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 card and select to "Pay". You will be asked to retry the payment and an error should be shown somewhere in the UI. The expected error string is described in each individual test. If you see the error, hit "Pay" again. If you don't see the error, abort the payment request by hitting "esc" - which means that particular test has failed.