message: Add run_exhaustively_while convenience function
diff: @@ -578,6 +578,12 @@ struct test_coordinator_fixture {
  sched.run_dispatch_loop(streaming_cycle);
  }
  + /// Dispatches messages and timeouts until no activity remains.
  + template <class Predicate>
  + void run_exhaustively_while(Predicate predicate) {
  + sched.run_dispatch_loop(predicate, streaming_cycle);
  + }
  +
  template <class T = int>
  caf::expected<T> fetch_result() {
  caf::expected<T> result = caf::error{};
  