Reapply "tests: disable flaky check on X11"

This reverts commit 8afffe6.

Author: amezin

tests/test_wm.py

@@ -665,7 +665,7 @@ def test_mouse_resize(
 
             wait_idle()
 
-            if shell_dbus_interface.ShellVersion[0] != 43:
+            if gdk_backend != GdkBackend.X11 and shell_dbus_interface.ShellVersion[0] != 43:
                 assert extension_test_hook.WindowRect == expected_rect2
 
         finally: