How to test in app purchases in Flutter [IAP][testing]

I'm using the official in_app_purchase package from Google, but I can't find any information on how to test IAP in flutter after it's set up.

Android and iOS has there own guides, but they require you to add libraries and code manually which is not compatible with the way flutter autogenerates native code.

Is there a "right" way of testing in app purchases in flutter?