The test aims to test that trying to fetch X11 clipboard content after
Xwayland went away doesn't cause issues. What happens though is that
sometimes the clipboard content doesn't have time to settle (i.e. fetch
mime types etc) before Xwayland gets terminated, which causes flakyness.
Fix this by waiting for the compositor side clipboard owners to finish
setting up before continuing.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2484>
It works by using an X11 client to set the clipboard content, using a
mimetype that on purpose is not handled by the clipboard manager. The
test then makes sure we don't crash when trying to transfer data from
the old X11 selection source.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2364>
