tests/clutter/event-delivery: Avoid race with stage update
clutter_actor_destroy() queues a stage update. Under certain
circumstances - i.e. when run in a very slow container - this can race
with the stage update triggered by the following
clutter_virtual_input_device_notify_button(), occasionally resulting in
wait_stage_updated() to return before the
on_event_return_propagate() callbacks ran, making the test fail.
This notably became more common since
8f27ebf8 (clutter/frame-clock: Start next update ASAP after idle period)
landed.
Thus wait for a stage update to happen after clutter_actor_destroy(),
preventing the race.
Fixes: f6da583d (tests/clutter/event-delivery: Add tests for implicit grabbing)
cc @daenzer