tests: temporarily disable GDBus async proxy test

It has never worked properly and, with the new GDBus testing stuff, it's
occasionally failing.

https://bugzilla.gnome.org/show_bug.cgi?id=672248

gio/tests/gdbus-proxy.c

@@ -796,6 +796,8 @@ proxy_ready (GObject      *source,
 static void
 test_async (void)
 {
+  return; /* XXX: See bug #672248 */
+
   g_dbus_proxy_new_for_bus (G_BUS_TYPE_SESSION,
                             G_DBUS_PROXY_FLAGS_NONE,
                             NULL,                      /* GDBusInterfaceInfo */