Fix python CLI CI test

[Lestropie]

Testing whether the proposed change in #2845 facilitates executing the Python CLI unit tests in CI for #2678.

I do realise in retrospect that this solution might not actually live that long (even if it's a useful capability to leave in place): with #2741, the Python CLI test would need to have the same mechanism of executable binary generation.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[Lestropie]

d9587e6 got the new Python CLI test working on both Unix and MacOS, but the Windows CI test fails, with command testing_python_cli unable to find the mrtrix3 Python module.

[Lestropie]

On Linux:

sys.stderr.write(repr(os.environ))
[..., 'PYTHONPATH': '/home/unimelb.edu.au/robertes/src/mrtrix3/build_cmake/lib/', ...]
sys.stder.write(repr(sys.path))
[..., '/home/unimelb.edu.au/robertes/src/mrtrix3/build_cmake/lib', ...]

On MSYS2:

sys.stderr.write(repr(os.environ))
[..., 'PYTHONPATH': 'C:/msys64/home/rob/src/mrtrix3_cmake/build_cmake/lib', ...]
sys.stder.write(repr(sys.path))
[..., '/home/rob/src/mrtrix3_cmake/testing/data/C', '/msys64/home/unimelb.edu.au/robertes/src/mrtrix3/build_cmake/lib', ...]

Setting PYTHONPATH via ENVIRONMENT is being done with ${PROJECT_BINARY_DIR}; this seems to be yielding an absolute path from the drive root with Windows-style drive separator; this gets corrupted when python reads it and appends sys.path.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[Lestropie]

Looks like my attempted hack fix didn't work; @daljit46 this might need your expertise.

[daljit46]

Differences between path handling between Windows and Unix systems is always a pain. Fortunately, MSYS2 ships with a tool called cygpath which allows you to convert between the different formats (if you look at the code in cmake/BashTests.cmake you can see that we're using this tool already for exec_dirs).

For this specific case, I think it's best to set the environment conditionally based on the OS. We could add some logic to do this automatically in BashTests.cmake but since the ENVIRONMENT property doesn't need to restrict itself to file paths only it's probably best not to.

So I propose the following:

set(PYTHON_PATH "${PROJECT_BINARY_DIR}/lib")
# On MSYS2 we need to convert Windows paths to Unix paths
if(MINGW AND WIN32)
      EXECUTE_PROCESS(
          COMMAND cygpath -u ${PYTHON_PATH }
          OUTPUT_VARIABLE PYTHON_PATH
          OUTPUT_STRIP_TRAILING_WHITESPACE
      )
endif()

# Then set environment
add_bash_tests(
    ...
    ENVIRONMENT "PYTHONPATH=${PYTHON_PATH}"
)

This should do the job (seems to work on my local MSYS2 installation)

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"