Fix the payment stubbing, payment `process_offline!` was stubbed to
return true and payment `completed?` was also stubbed to return true.
`process_offline!` did not change the payment status in the database,
meaning that any call to 'order.pending_payments' after that would
return the payment which should have been processed offline, and
therefore completed. This created some unwanted side effect, resulting
in the test breaking.
5fd36e7eed