debug-only issue. the test was doing

  set debug_sync='now SIGNAL go3';
  ...
  set debug_sync='reset';

which translated into

  add "go3" to the hash of active signals
  pthread_broadcast to wake up waiting threads
  ...
  clear the hash of active signals

as a result a waiting thread was awoken, but the hash was emptied
before the thread checked if its signal was in the hash. so the
thread didn't find its signal and went back to sleep.

let's wait until the awoken thread has completely finished
disconnecting and was added to the thread cache.