[libcxx testing] Fix UB in tests for std::lock_guard

If mutex::try_lock() is called in a thread that already owns the mutex,
the behavior is undefined. The patch fixes the issue by creating another
thread, where the call is allowed.

Differential Revision: https://reviews.llvm.org/D94656

GitOrigin-RevId: 78036360573c35ea9e6a697d2eed92db893b4850
2 files changed