main: Warn instead of error in meta_test_init()

Otherwise we'll get the warning

../src/core/main.c: In function 'meta_test_init':
../src/core/main.c:755:1: error: function might be candidate for attribute 'noreturn' [-Werror=suggest-attribute=noreturn]
  755 | meta_test_init (void)
      | ^~~~~~~~~~~~~~

when building without Wayland.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/837

src/core/main.c

@@ -766,6 +766,6 @@ meta_test_init (void)
 
   close (fd);
 #else
-  g_error ("Tests require wayland support");
+  g_warning ("Tests require wayland support");
 #endif
 }