fs-mistrust: Fix test compilation on windows.

std::os::unix::fs::MetadataExt was indeed the right trait to import, but it doesn't exist on non-unix platforms.

This is another bugfix on !2707 (merged). It should retain the fix of !2717 (merged). I've confirmed that it builds on Windows and passes tests on Linux and Mac.

Assigning to @gabi-250, since you reviewed !2717 (merged) and hopefully still have this logic paged in. :)