If you're just testing new beta features, you don't need to make a purchase at all. See: Do I need a purchase to use the beta?
If you'd like to use the regular (non-beta) features of the app, then you do need a purchase. Temporarily, there's no way to make a purchase in the iOS beta. You'd have to delete the beta app, get the regular app from the App Store (not TestFlight), make a purchase in the regular app, and then get back to the beta. We're working on making this process easier.