[libcxx] [test] Fix filesystem permission tests for windows

On Windows, the permission bits are mapped down to essentially only
two possible states; readonly or readwrite. Normalize the checked
permission bitmask to match what the implementation will return.

Differential Revision: https://reviews.llvm.org/D101728

NOKEYCHECK=True
GitOrigin-RevId: 68de58cd649cb3a3e94a1c9552ebf2a18bb9d040
3 files changed