Provide rendezvous with timeout to avoid deadlock in tests

Using the rendezvous built-in function can lead to deadlock if not all threads arrive at the rendezvous point. This can cause tests to hang, provide little diagnostic information.

It would be useful to be able to associate a timeout with a rendezvous, such that if the required number of threads did not arrive at the rendezvous before the timeout expired, a list of all arriving threads/not arriving threads is printed, together with a thread dump, together with an exception thrown so the test can exit cleanly.