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

[jieyouxu]

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"

[jieyouxu]

Error messages look like:

$ ./x test tests/ui/auto-instantiate.rs
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 1.67s
ERROR: no `test` rules matched ["E:\\repos\\rust\\tests\\ui\\auto-instantiate.rs"]
HELP: run `x.py test --help --verbose` to show a list of available paths
NOTE: if you are adding a new Step to bootstrap itself, make sure you register it with `describe!`
Build completed unsuccessfully in 0:00:04

$ ./x test tests\ui\auto-instantiate.rs
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
ERROR: no `test` rules matched ["testsuiauto-instantiate.rs"]
HELP: run `x.py test --help --verbose` to show a list of available paths
NOTE: if you are adding a new Step to bootstrap itself, make sure you register it with `describe!`
Build completed unsuccessfully in 0:00:01

$./x test tests\\ui\\auto-instantiate.rs
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
ERROR: no `test` rules matched ["E:\\repos\\rust\\tests\\ui\\auto-instantiate.rs"]
HELP: run `x.py test --help --verbose` to show a list of available paths
NOTE: if you are adding a new Step to bootstrap itself, make sure you register it with `describe!`
Build completed unsuccessfully in 0:00:01