test deadlock: test-threadpool sync-joinable

[ichthyo]

The sync-joinable test runs into a deadlock.

The situation is as follows:

• the main thread (running the test function) blocks waiting on the join()
• the 'master' thread blocks waiting on (test-threadpool, 99)

    lumiera_thread_sync_other (worker); // wait until the arguments are sent
  

• but the worker thread doesn't even start the thread function; it blocks in threads.c, 95

  static void*
  thread_loop (void* thread)
  {
  ...
  ...
    LUMIERA_CONDITION_SECTION (cond_sync, &t->signal)
      {
        t->rh = &lumiera_lock_section_.rh;
  
        do {
          lumiera_threadpool_release_thread(t);
          LUMIERA_CONDITION_WAIT (t->state != LUMIERA_THREADSTATE_IDLE);
          
  >>>^^^^^Waiting here        
  ...
  
          // NULL function means: no work to do
          INFO (threads, "function %p", t->function);
          if (t->function)
            t->function (t->arguments);