TESTING DEVICE AND METHOD FOR DETERMINING WATER PRESSURE IN FRACTURE OF WATER-INJECTED COAL SEAM TECHNICAL FIELD The present disclosure relates to the technical field of pore water pressure testing, in particular to a testing device and method for determining a water pressure in a fracture structure around a water injection borehole of a coal seam. BACKGROUND Coal seam water injection is a comprehensive mine disaster prevention technology commonly used in the world. After decades of practice and development, nearly 80% of fully mechanized mining faces in China have adopted the coal seam water injection technology. In recent years, with the continuous advancement, the application fields of the coal seam water injection technology are expanding, from dust control, prevention of rock bursts and prevention of coal and gas outbursts to elimination of toxic gases such as hydrogen sulfide in coal seams, as well as hard roof softening and top coal caving. In the process of coal seam water injection, the occurrence of gas, coal seam hardness, pore structure, wetting characteristics and the occurrence of water in the coal seam vary greatly with the water injection pressure. Therefore, timely adjustment of water injection pressure in the water injection process plays an important role in improving the disaster prevention effect of coal seam water injection. The prerequisite for adjusting the water injection pressure is to accurately obtain the water pressure in the fracture of the water-injected coal seam. The method for measuring the fluid pressure in the coal seam around the water injection borehole in the prior art includes: drilling a borehole, laying a pressure measuring pipe, sealing the borehole, installing a pressure gauge at the mouth of the borehole and reading. In this method, the pressure displayed by the pressure gauge is the combined pressure of air and water, and the water pressure cannot be accurately measured. SUMMARY Technical problem Solution to the problem Technical solution In order to solve the above-mentioned defects existing in the prior art, the present disclosure provides a testing device and method for determining a water pressure in a fracture of a water-injected coal seam. The present disclosure acquires the diameter of a water inlet in a test part through a field test. Then the present disclosure uses the diameter as an intermediate quantity to calculate a water pressure in a fracture structure around a coal seam water injection borehole according to a relationship between a capillary pore size and the water pressure in the Washburn's equation. An objective of the present disclosure is to provide a testing device for determining a water pressure in a fracture of a water-injected coal seam. The technical solution includes: A testing device for determining a water pressure in a fracture of a water-injected coal seam, including a testing mechanism and a display mechanism, where the testing mechanism includes a housing, a filter membrane, a permeable steel mesh, a water-absorbing swelling rubber, a positive terminal connector, a negative terminal connector and an explosion-proof power supply; the housing is cylindrical, and is provided thereon with a plurality of water inlets with different diameters; a plurality of partitions are provided in a cylindrical cavity inside the housing; the plurality of partitions divide the cylindrical cavity into several sealed cavities; a water-proof rubber tube is provided at the center of the cylindrical cavity; a line connected to the explosion-proof power supply is placed in the water-proof rubber tube; the water-absorbing swelling rubber is correspondingly placed in the sealed cavity; the permeable steel mesh and the filter membrane are sequentially arranged between the water-absorbing swelling rubber and the housing; the filter membrane is used to isolate impurities in the water; when the water-absorbing swelling rubber expands by absorbing water, the permeable steel mesh is used to protect the filter membrane and make the water-absorbing swelling rubber move to one side; the display mechanism includes an external wire, indicator lights and a switch; the external wire is connected to the line connected to the explosion-proof power supply; a wire is provided inside the water-absorbing swelling rubber, and one end of the wire is provided with a positive terminal connector; an expansion space is reserved between the water-absorbing swelling rubber and the sealed cavity; the explosion-proof power supply is adjacent to the expansion space; the negative terminal connector is located at the explosion-proof power supply; when the water-absorbing swelling rubber absorbs water and expands, the positive terminal connector is connected to the negative terminal connector, so that a closed current loop is formed between the explosion-proof power supply and the display mechanism, and an indicator light of the display mechanism turns on. Preferably, the partitions are welded in the cylindrical cavity. Further preferably, the filter membrane is a reverse osmosis (RO) membrane. Another objective of the present disclosure is to provide a testing method for determining a water pressure in a fracture of a water-injected coal seam by using the testing device described above, where the testing method includes the following steps: a. designing the number of the sealed cavities and the size of corresponding water inlets, and constructing the sealed cavities and water inlets according to a coal seam water injection scheme; constructing a plurality of water pressure test holes around a water injection borehole after the coal seam water injection borehole is drilled; b. connecting the testing mechanism with the display mechanism before coal seam water injection starts, determining a connection sequence of the sealed cavities with the indicator lights, verifying the reliability of a circuit, pushing the testing mechanism into the bottom of the test borehole after the verification is completed, and sealing the borehole; c. allowing low-pressure water to enter a low-pressure sealed cavity through larger water inlets and high-pressure water to enter a high-pressure sealed cavity through smaller water inlets after coal seam water injection starts, where the water inlets of different diameters represent different sizes of capillaries; observing the indicator lights of the display mechanism, and measuring a relevant parameter after an indicator light turns on; then calculating the water pressure by the diameter of the water inlets in the sealed cavity according to a Washburn's equation (1): 2ucosO r (1) where, in Eq. (1), P represents a capillary pressure corresponding to the diameter r; u and
O represent a surface tension and a contact angle of the liquid, respectively; d. adjusting a water injection parameter in time according to a calculation result after the water pressure calculation is completed; and e. recovering the display mechanism for recycling after the water pressure test is completed. Beneficial effects of the present disclosure Beneficial effects The present disclosure has the following beneficial technical effects: When there are two phases of gas and water in the coal seam fracture, the water can overcome the gas pressure and capillary resistance to enter the smaller capillary structure only after reaching a certain pressure. Based on this, the present disclosure designs a water pressure testing device. A housing of a testing mechanism is provided thereon with water inlets with different diameters representing different sizes of capillaries. Low-pressure water enters a low-pressure sealed cavity inside the testing device through larger water inlets, and high-pressure water enters a high-pressure sealed cavity through smaller water inlets. A RO filter membrane only allows water molecules to pass through, and isolates impurities such as sand in the water. After the water enters a sealed cavity, a water-absorbing swelling rubber expands under the action of water. As a permeable steel mesh is provided outside the water-absorbing swelling rubber, the water-absorbing swelling rubber in the sealed cavity can only move to one side of the device. After it moves to a certain extent, positive and negative terminal connectors of a power supply in the sealed cavity are connected, so that the internal explosion-proof power supply and an external display mechanism form a closed current loop, and an indicator light on the external display mechanism corresponding to the sealed cavity turns on. Then, according to the relationship between the capillary pore size and water pressure in the Washburn's equation, the water pressure in the fracture structure around the coal seam water injection borehole is calculated. The device of the present disclosure can accurately and quantitatively calculate the water pressure in the coal seam around the borehole during water injection, and the calculation result has an important effect on adjusting the water injection parameters and improving the comprehensive disaster prevention effect of water injection. BRIEF DESCRIPTION OF THE DRAWINGS Description of the drawings The present disclosure is explained in detail below with reference to the accompanying drawings. FIG. 1 is a structural view of a testing mechanism according to the present disclosure. FIG. 2 is a sectional view of the testing mechanism according to the present disclosure. FIG. 3 is a structural view of a display mechanism according to the present disclosure. Reference Numerals: 1. housing; 2. larger water inlet; 3. reverse osmosis (RO) filter membrane; 4. permeable steel mesh; 5. water-absorbing swelling rubber; 6. positive terminal connector; 7. negative terminal connector; 8. smaller water inlet; 9. wire; 10. sealed cavity; 11. explosion-proof power supply; 12. power supply cavity; 13. water-proof rubber tube; 14. partition; 15. indicator light; 16. external wire; and 17. switch. DETAILED DESCRIPTION Detailed description of the present disclosure The present disclosure provides a testing device and method for determining a water pressure in a fracture of a water-injected coal seam. In order to make the advantages and technical solutions of the present disclosure more clearly, the present disclosure is described in detail below with reference to the specific embodiments. As shown in FIGS. 1 to 3, a testing device for determining a water pressure in a fracture of a water-injected coal seam includes a testing mechanism and a display mechanism. The testing mechanism includes a housing 1, larger water inlets 2, a reverse osmosis (RO) filter membrane 3, a permeable steel mesh 4, a water-absorbing swelling rubber 5, a positive terminal connector 6, a negative terminal connector 7, smaller water inlets 8, a wire 9, a sealed cavity 10, an explosion-proof power supply 11, a power supply cavity 12, a water-proof rubber tube 13 and partitions 14. The housing 1 is cylindrical, and is provided thereon with a plurality of water inlets with different diameters, namely, the larger water inlets 2 and the smaller water inlets 8. A plurality of partitions 14 are provided in a cylindrical cavity inside the housing. The plurality of partitions 14 divide the cylindrical cavity into several sealed cavities 10. The water-proof rubber tube 13 is provided at the center of the cylindrical cavity. A line connected to the explosion-proof power supply is placed in the water-proof rubber tube. The display mechanism includes indicator lights 15, an external wire 16 and a switch 17. There are a plurality of indicator lights 15 which are respectively connected to different sealed cavities during testing. The housing 1 is made of a high-strength steel pipe. The plurality of water inlets with different diameters represent capillaries of different sizes. When there are two phases of gas and water in the fracture, the water can overcome the gas pressure to enter the smaller capillary structure only after reaching a certain pressure. Therefore, low-pressure water enters a low-pressure sealed cavity through the larger water inlets 2, and high-pressure water enters a high-pressure sealed cavity through the smaller water inlets 8. The sealed cavities are formed by the partitions 14 welded in the cylindrical cavity. The RO filter membrane 3 and the permeable steel mesh 4 are respectively provided outside the sealed cavities. The RO filter membrane only allows water molecules to pass through and isolates impurities such as sand in the water. The permeable steel mesh has a strength to protect the RO filter membrane and prevent the water inlets from being blocked when the water-absorbing swelling rubber expands, and ensure that the water-absorbing swelling rubber 5 in the sealed cavity can only move to one side of the device. The water-absorbing swelling rubber has high elasticity and good mechanical strength, and can expand several times to several hundred times after absorbing water. A wire is provided inside the water-absorbing swelling rubber, and one end of the wire is provided with a positive terminal connector. After the water-absorbing swelling rubber reacts with the water, the power supply positive and negative terminal connectors in the sealed cavity are connected to form a closed current loop between the internal explosion-proof power supply and the external display mechanism, and an indicator light on the external display mechanism corresponding to the sealed cavity turns on After the indicator light turns on, according to the relationship between the capillary pore size and water pressure in the Washburn's equation, the water pressure in the fracture structure around the coal seam water injection borehole is calculated. A testing method for determining a water pressure in a fracture of a water-injected coal seam by using the testing device includes the following steps: a. Design the number of the sealed cavities and the size of corresponding water inlets, and construct the sealed cavities and water inlets according to a coal seam water injection scheme; construct a plurality of water pressure test holes around a water injection borehole after the coal seam water injection borehole is drilled. b. Connect the testing mechanism with the display mechanism before coal seam water injection starts, determine a connection sequence of the sealed cavities with the indicator lights, and verify the reliability of a circuit; use a drill rod to push the testing mechanism into the bottom of the test borehole after the verification is completed, and seal the borehole. c. Allow low-pressure water to enter a low-pressure sealed cavity through larger water inlets and high-pressure water to enter a high-pressure sealed cavity through smaller water inlets after coal seam water injection starts, where the water inlets of different diameters represent different sizes of capillaries; observe the indicator lights of the display mechanism, and measure a relevant parameter after an indicator light turns on; then calculate the water pressure by the diameter of the water inlets in the sealed cavity according to a Washburn's equation (1): 2c cosO r (1)
In Eq. (1), P represents a capillary pressure corresponding to the diameter r; u and 0
represent a surface tension and a contact angle of the liquid, respectively. d. Adjust a water injection parameter in time according to a calculation result after the water pressure calculation is completed, for example, when the test water pressure is too small, increase the water injection pressure in time. e. Recover the display mechanism for recycling after the water pressure test is completed. Those not mentioned in the present disclosure may be realized by adopting or learning from the prior art. Although terms such as RO filter membrane 3, permeable steel mesh 4 and water-absorbing swelling rubber 5 are frequently used in the present disclosure, the possibility of using other terms is not excluded. The terms are only intended to describe and explain the essence of the present disclosure more conveniently, and it is contrary to the spirit of the present disclosure to interpret these terms as an additional limitation. It should be further explained that the specific embodiments described herein are merely intended to illustrate the spirit of the present disclosure. A person skilled in the art may make various modifications or supplements to the specific embodiments described or replace them in a similar manner, but it may not depart from the spirit of the present disclosure or the scope defined by the appended claims.