Add `sync_effects_completed()` and `verify_view()` in
order to allow Wayland test clients to trigger verifications
and add convenience functions to use them to client-utils.
Notes:
- `sync_effects_completed()` works in two stages in order
to ensure it doesn't race with window effects. By the time
`sync_effects_completed()` is processed, an effect could
already have ended or not yet been scheduled. Thus we
defer a check for pending effects to the next paint cycle,
assuming that by then they should have been scheduled.
- `meta_ref_test_verify_view()` internally triggers the
`paint` signal for the stage which is why it can not be run
in the after-paint signal handler.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1055>
