fix: Replace gtest_discover_tests for Visual Studio 2022 builds

[RobBuchananCompPhys]

DIssolve GTest unit tests have been failing to build in our Visual Studio 2022 (MSVC) setup. The failure appears to be related to a timeout at the link step* when GTest is attempting to run a test executable. I have found that swapping out the call to gtest_discover_tests with the lower-level CTest add_test gets around this hang, allow the tests to proceed to link and allows for running and debugging.

*See the example below:

  CMake Error at C:/Program Files/Microsoft Visual Studio/2022/Community/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.31/Modules/GoogleTestAddTests.cmake:132 (message):
    Error running test executable.
  
      Path: 'C:/Users/vcb31993/source/repos/disorderedmaterials/dissolve/build/bin/nodes_graph_argon.exe'
      Working directory: 'C:/Users/vcb31993/source/repos/disorderedmaterials/dissolve/tests/data'
      Result: Process terminated due to timeout
      Output: