libstdc++: Fix testsuite helper functions [PR 97936]

libstdc++: Fix testsuite helper functions [PR 97936]

This fixes a race condition in the util/atomic/wait_notify_util.h header
used by several tests, which should make the tests work properly.

libstdc++-v3/ChangeLog:

	PR libstdc++/97936
	* testsuite/29_atomics/atomic/wait_notify/bool.cc: Re-eneable
	test.
	* testsuite/29_atomics/atomic/wait_notify/generic.cc: Likewise.
	* testsuite/29_atomics/atomic/wait_notify/pointers.cc: Likewise.
	* testsuite/29_atomics/atomic_flag/wait_notify/1.cc: Likewise.
	* testsuite/29_atomics/atomic_float/wait_notify.cc: Likewise.
	* testsuite/29_atomics/atomic_integral/wait_notify.cc: Likewise.
	* testsuite/util/atomic/wait_notify_util.h: Fix missed
	notifications by making the new thread wait until the parent
	thread is waiting on the condition variable.