Fast way to test (manually) commerce checkout process
This is not specific to Drupal, but you can use Selenium IDE.

Selenium is a framework for automated testing, but the IDE allows you to do lazy manual testing too.

You need to install Firefox and the Selenium IDE addon.

Basic explanation of how to get started


Open the IDE (addon) and press the red "record" button.
Go to your site and carry out all the steps manually. Selenium IDE will automatically record the steps as you go along.
You can also add steps manually or edit the steps in the displayed editor.  This supports cut and paste so you can edit it pretty quickly.
When you're done, save the file.  You can then open this file again in the IDE and press the play button to automatically carry out all the steps.