x.py test specific directory requires platform specific path separator

[Havvy]

On Windows, I tried this code:

python x.py test src/test/ui/expr/compound-assignment/eval-order.rs --pass run

I expected to execute the file and give back a test result. Instead, it found zero results.

After an hour of flailing, I tired:

python x.py test src/test/ui\expr\compound-assignment\eval-order.rs --pass run

And to my (pleasant) surprised, it found the file.

Ideally, it shouldn't matter which path separator is used at any point.

[jyn514]

This is an issue with libtest - compiletest passes in expr/compound-assignment/eval-order.rs as the filter, which libtest counts as filtering out expr\compound-assignment\eval-order.rs too.

I think the workaround is to manually turn forward slashes into backslashes in impl Step for test::Compiletest when running on windows.

[czzrr]

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