libstdc++: Fix some more deadlocks in tests [PR 97936]

libstdc++: Fix some more deadlocks in tests [PR 97936]

The missed notifications fixed in r11-5383 also happen in some other
tests which have similar code.

libstdc++-v3/ChangeLog:

	PR libstdc++/97936
	* testsuite/29_atomics/atomic/wait_notify/bool.cc: Fix missed
	notifications by making the new thread wait until the parent
	thread is waiting on the condition variable.
	* testsuite/29_atomics/atomic/wait_notify/pointers.cc: Likewise.
	* testsuite/29_atomics/atomic_flag/wait_notify/1.cc: Likewise.
	* testsuite/29_atomics/atomic_ref/wait_notify.cc: Likewise.