According to this helpful page:
Test an Interrupted Transaction
Set a breakpoint in your transaction queue observer’s paymentQueue:updatedTransactions: method so you can control whether it delivers the product. Then make a purchase as usual in the test environment, and use the breakpoint to temporarily ignore the transaction—for example, by returning from the method immediately using the thread return command in LLDB. Terminate and relaunch your app. Store Kit calls the paymentQueue:updatedTransactions: method again shortly after launch; this time, let your app respond normally. Verify that your app correctly delivers the product and completes the transaction.
Hope this helps someone.