context/test: Add 'no-x11' and 'test-client' constructor flags

The 'no-x11' one will inhibit Xwayland from starting, and 'test-client'
will make sure the test client path is properly discovered.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>

src/tests/stage-view-tests.c

@@ -1161,7 +1161,8 @@ main (int argc, char *argv[])
 {
   g_autoptr (MetaContext) context = NULL;
 
-  context = meta_create_test_context (META_CONTEXT_TEST_TYPE_NESTED);
+  context = meta_create_test_context (META_CONTEXT_TEST_TYPE_NESTED,
+                                      META_CONTEXT_TEST_FLAG_NO_X11);
   g_assert (meta_context_configure (context, &argc, &argv, NULL));
 
   init_tests ();