Manual Tests for PaymentResponse.complete() - Please run in order!

Click on each button in sequence from top to bottom without refreshing the page. Each button will bring up the Payment Request UI window.

When presented with the payment sheet, use any credit card select to "Pay". Also confirm any prompts that come up.

1. If the value of the internal slot [[completeCalled]] is true, reject promise with an "InvalidStateError" DOMException.
2. Passing no argument defaults to "unknown", eventually closing the sheet and doesn't throw.
3. Passing "success" eventually closes the sheet and doesn't throw.
4. Passing "fail" eventually closes the sheet and doesn't throw.
5. Passing "unknown" eventually closes the sheet and doesn't throw.
6. Done!