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/test-runner.c

@@ -797,7 +797,6 @@ main (int argc, char **argv)
   g_option_context_free (ctx);
 
   meta_plugin_manager_load (test_get_plugin_name ());
-  meta_wayland_override_display_name ("mutter-test-display");
 
   meta_init ();
   meta_register_with_session ();