`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
8f27ebf87e (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: f6da583d06 (tests/clutter/event-delivery: Add tests for implicit grabbing)
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3332>
b5a59ec109