context/test: Respect MUTTER_SYNC envvar

It just calls meta_set_syncing() accordingly. Is currently done by
meta_init() either when the env var is set, or when a command line
argument is passed.

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

src/tests/meta-context-test.c

@@ -91,6 +91,8 @@ meta_context_test_setup (MetaContext  *context,
     settings,
     META_EXPERIMENTAL_FEATURE_SCALE_MONITOR_FRAMEBUFFER);
 
+  meta_set_syncing (!!g_getenv ("MUTTER_SYNC"));
+
   return TRUE;
 }