[libcxx] [test] Make some threading tests more robust

Increase the timeout tolerance if TEST_IS_EXECUTED_IN_A_SLOW_ENVIRONMENT
is set, similarly to how it's done in a couple other tests.

Use `std::this_thread::yield();` instead of busylooping. When multiple
threads are busylooping, it's plausible that not all threads even get
started running before the timeout runs out.

This makes the threading tests succeed if run in Windows runners on
Github Actions.

Differential Revision: https://reviews.llvm.org/D131483