tests/respect-constraints: Explicitly set monitor size

The test depends on a certain monitor size; make sure to set that up
front.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2554>

src/tests/stacking/client-resize-respect-constraints.metatest

@@ -4,6 +4,8 @@
 #  3. Resize such that the following resize will extend beyond the screen
 #  4. Check that the window was moved to the position that would allow it to fit on the screen
 
+resize_monitor primary 800 600
+
 new_client w wayland
 create w/1 csd