This test aims at checking that the transient relationship set using the
xdg-foreign API is respected by mutter and that no crash occurs, such as
the one in issue !174.
Note: the crash from issue !174 occurs only if "attach_modal_dialogs" is
set, so one has to change that default value to "true" to be able to
trigger the crash:
gsettings set org.gnome.mutter attach-modal-dialogs true
Related: https://gitlab.gnome.org/GNOME/mutter/issues/174
