`./x test <path_to_test>` does not work on msys2

Currently, the run-make test suite on windows still can only run in msys2.

When trying to use ./x test <path_to_specific_test> inside the MSYS2 MSYS program, the usual test path/filters that works on other platforms (including outside of msys2 but in powershell or cmd) does not work within MSYS2 MSYS. This is not specific to the run-make test suite, but run-make tests are mentioned because they caused the requirement of msys2 in the first place.

For example, for a test tests/run-make/CURRENT_RUSTC_VERSION:

Examples that doesn't work

These do work outside MSYS2 MSYS on windows (for backslash path separators) and on other platforms, but not within MSYS2 MSYS:

./x test tests/run-make/CURRENT_RUSTC_VERSION
./x test tests\\run-make\\CURRENT_RUSTC_VERSION

This doesn't work in MSYS2 MSYS either

./x test tests\run-make\CURRENT_RUSTC_VERSION

Examples that do work

This incantation does work, but it is annoying to type

./x test run-make --test-args="tests/run-make/CURRENT_RUSTC_VERSION"

Start by reproducing the issue with ./x test tests/run-make/CURRENT_RUSTC_VERSION in the MSYS2 MSYS program, then compare it with the working run-make --test-args form. Trace how ./x test <path> handles test paths and MSYS2 path separators. Done means a specific test path works directly in MSYS2 without the workaround.