`./x test --skip` is sensitive to path separators on Windows

[ChrisDenton]

Currently the following will not skip any tests on Windows even though the test path exists

./x test ui --skip tests/ui/traits/fn-pointer/bare-fn-no-impl-fn-ptr-99875.rs

This is because of using / instead of \ for the path separators. Ideally bootstrap should handle this.

Additionally it would be nice if --skip errored (or at least warned) if the given path didn't match anything.

[jieyouxu]

Reopening because this is a bit more tricky, as Steps may register arbitrary aliases and there is also logic that accepts matching prefix/suffixes for tests (see #134843 for more details).

[onur-ozkan]

The fix doesn't have anything to do with aliases.