How do you actually test "Ask to buy"?

I'm aware of simulatesAskToBuyInSandbox, and that first part seems to work well enough, but I'm at a loss as to what to do after that – I know there's no way to test the parent's side of that (since there's no way to set up a family relationship with sandbox accounts).

The part I'm stuck on is testing the app's response to the approval / denial of the purchase, and I can't find any information on how to trigger that, if it's even possible.

Is it actually necessary to test in the live environment, with real money?