I don't get it. Surely the point of testing mode is so we can get test ads to ensure our flow is working correctly during development? I cannot get a single interstitial when using testing mode. This is true for both the previous SDK and the new SDK.
Did you happen to be testing on an iPad? There was no test ad being served to iPads at the time. I have enabled a test ad that should serve to all devices, now, so could you give it another try and let me know if you get test Interstitial Ads, now?