tests: Add the "parent_exported" test

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

src/Makefile-tests.am

@@ -17,6 +17,7 @@ dist_stacking_DATA =						\
 	$(srcdir)/tests/stacking/minimized.metatest 	\
 	$(srcdir)/tests/stacking/mixed-windows.metatest     \
 	$(srcdir)/tests/stacking/set-parent.metatest	\
+	$(srcdir)/tests/stacking/set-parent-exported.metatest	\
 	$(srcdir)/tests/stacking/override-redirect.metatest
 
 mutter-all.test: tests/mutter-all.test.in

src/tests/stacking/set-parent-exported.metatest

@@ -0,0 +1,15 @@
+new_client 1 wayland
+create 1/1
+show 1/1
+create 1/2
+show 1/2
+wait
+
+set_parent_exported 1/1 2
+wait
+assert_stacking 1/2 1/1
+
+local_activate 1/2
+assert_stacking 1/2 1/1
+
+destroy 1/2