tests/stacking: Add test hiding a modal with a not shown parent

This adds a test case for
https://gitlab.gnome.org/GNOME/mutter/-/issues/862 that checks that
hiding a dialog where its parent is not yet shown doesn't trigger any
asserts or crashes.

(cherry picked from commit c94d9293)

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1650>