message: Avoid deadlock in group module with test scheduler
diff: @@ -54,6 +54,8 @@ void await_all_locals_down(actor_system& sys, std::initializer_list<actor> xs) {
  self->send_exit(x, exit_reason::kill);
  ys.push_back(x);
  }
  + // Don't block when using the test coordinator.
  + if (sys.config().scheduler_policy != atom("testing"))
  self->wait_for(ys);
  }
  