tests: Rename test-utils.c/h to meta-test-utils.c/h

Otherwise it'll conflict with other files with identical filenames,
if one would add src/tests/ to the include path, which will happen in a
later commit.

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

src/tests/monitor-unit-tests.c

@@ -32,7 +32,7 @@
 #include "meta-backend-test.h"
 #include "tests/meta-monitor-manager-test.h"
 #include "tests/monitor-test-utils.h"
-#include "tests/test-utils.h"
+#include "tests/meta-test-utils.h"
 #include "x11/meta-x11-display-private.h"
 
 static MonitorTestCase initial_test_case = {