Unfortunately there isn't. Ideally, submitting a test request would propagate the test flag to the mediation networks, but AdMob currently goes through a different flow for a test request, and mediation never kicks off. Providing better testing mechanisms, including this capability, is on our road map, but it won't be available for a while. This affects both Android and iOS by the way.
Side note: The testing property on iOS is deprecated. See
https://developers.google.com/mobile-ads-sdk/docs/admob/additional-controls#testmode for how to properly set test devices on your GADRequest.
Thanks,
Eric