tests: Override Wayland display name in test_init()

It's overridden by all test cases, but they all also call test_init() so
do it there instead.

https://gitlab.gnome.org/GNOME/mutter/issues/193

src/tests/unit-tests.c

@@ -263,7 +263,6 @@ main (int argc, char *argv[])
 
   meta_override_compositor_configuration (META_COMPOSITOR_TYPE_WAYLAND,
                                           META_TYPE_BACKEND_TEST);
-  meta_wayland_override_display_name ("mutter-test-display");
 
   meta_init ();
   meta_register_with_session ();