Add tests for pthread primitives inducing synchronization

[RalfJung]

Basically, something similar to #2270 for pthread mutex, rwlock, and condvar (but we can use deterministic scheduling by disabling preemption). Commenting out any of the validate_lock_release, validate_lock_release_shared, validate_lock_acquire in src/sync.rs should give a data race error in some test, and that does not currently happen, at least not for rwlock and condvar -- I assume that's because the standard library has some atomic variables on its own here that induce extra synchronization. So we probably need tests that directly talk to these APIs. (Also note that the pthread primitives are not even used on Linux any more, only on macOS.)

[RalfJung]

#3296 does this for mutex and rwlock. It is still pending for other pthread_cond.