tests: Avoid g_assert() use in unit tests

This is aimed towards allowing g_assert() to be compiled out of
release builds by ensuring they do not affect the testsuite.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4011>

src/tests/native-kms-headless-start.c

@@ -138,7 +138,7 @@ main (int argc, char *argv[])
 
   context = meta_create_test_context (META_CONTEXT_TEST_TYPE_VKMS,
                                       META_CONTEXT_TEST_FLAG_NO_X11);
-  g_assert (meta_context_configure (context, &argc, &argv, NULL));
+  g_assert_true (meta_context_configure (context, &argc, &argv, NULL));
 
   init_tests ();