[libc++] Sporadic assertion failures from `shared_mutex` tests on Windows

In recent months, several tests in `libcxx/std/thread/thread.mutex/thread.mutex.requirements/thread.sharedtimedmutex.requirements/thread.sharedtimedmutex.class` have been sporadically failing on Windows. The following tests sometimes don't work as expected.

- `try_lock_for.pass.cpp`
- `try_lock_until.pass.cpp`
- `try_lock_shared_for.pass.cpp`
- `try_lock_shared_until.pass.cpp`

Start by reviewing the sharedtimedmutex.class test directory and the four named files: try_lock_for.pass.cpp, try_lock_until.pass.cpp, try_lock_shared_for.pass.cpp, and try_lock_shared_until.pass.cpp. Run these tests on Windows and investigate the sporadic assertion failures; done means the affected tests reliably pass without weakening their assertions.

Written by the indexing model from the issue text.