Flushing the input thread might implicitly iterate the mainloop, and thus
update the stage while still inside the clutter_test_flush_input() call.
This means the stage update has already happened when we call
wait_stage_updated(), and that's why we call clutter_stage_schedule_update()
there currently.
This clutter_stage_schedule_update() call is not necessary though, instead
we can flush the input thread from inside wait_stage_updated() after
setting was_updated to FALSE.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2792>
57ce4e04e5