Make subscriptions work in test mode#29

It would be super useful to be able to test subscriptions (and trials) in test mode like one-time purchases. Not only being able to subscribe, but also advancing subscriptions in time to test headless integrations.

Right now, the only way to do this is to have a live store, a daily subscription and wait for cycles to happen. This really slows down integration validation, especially for people like me who don’t want to create test users in production (don’t want to pollute production data with test data).

Having this would make test mode pretty much feature complete too!