tst_QWaitCondition: Prevent test functions from interfering with each other.

Introduce a base class for the threads that ensures termination in the
destructor to ensure all QThreads instantiated on the stack are terminated.
This should reduce crashes since the test thread classes have pointers
to stack variables of the test slots.

Set object names on the threads for better diagnostics.

Decouple wakeOne()/wakeAll() that impact each other via the static count
variables of the thread class by introducing a base class WakeThreadBase
keeping a pointer to an QAtomicInt count variable on the stack instead
(similar to the existing pointers to the mutexes, etc).

Task-number: QTBUG-49653
Change-Id: I73537386bf36019efa81e8e24ba9af92506f7794
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>