The test case problem stemmed from the fact that a debug sync
signal is a global variable that persists until overwritten
by a new signal. This means that if two different signals
are raised in sequence, a thread waiting for the first signal
might miss it if the second signal sets the global variable
before the thread wakes up.

The solution is to deliver a subsequent signal only after the
waiting thread has received it.

mysql-test/t/query_cache_debug.test:
  Wait for signal to be delivered.