Had the same problem. I contacted as well and got this response from Google
Thank you for contacting Google Play Developer Support and reporting the behavior you’re seeing with in-app billing.
We recently made some changes to our systems and we are now requiring an app to be published before testing. We are currently recommending to publish your APK to the Alpha channel in order to test licensing, in-app billing, and expansion files. There is no need to create a special testing group in the Alpha channel to test these features, however the app must be published and not in draft mode.
We apologize for the inconvenience and are working to update our documentation to reflect these changes.
After publishing my APK to Alpha (as I haven’t launched yet) and waiting an hour or so, my IAP test purchases started working again.