The fastest way to test a checkout page is to use the following sample Visa number and fill in fake text for the remaining fields.
Here are other card types that you might want to test:
American Express: 378282246310005
Simulating Credit Card Errors
Novi AMS has some built in validation to ensure that fields are not missed and that expired cards are not used.
If you would like to go further, you can also simulate certain errors invalid card numbers or a declined card. For QuickBooks Payments, the way to emulate failures is to use one of the following in the "Name on Card" field. You will still need to enter fake data (including a credit card number) into the other fields.
Payment system error: "emulate=10201"
Card number is invalid: "emulate=10301"
General decline: "emulate=10401"
There is more documentation on the Intuit Developer website.