Setup proper testing harness

[elia]

We should both provide the users and use ourselves reliable testing harness. This can take the form of a bogus gateway or pre-recorded VCR scenarios.

The support should include:

• onboarding
• usage from the solidus checkout
• usage from the solidus cart page
• usage from the product page

[elia]

Moved this one out of the main project workflow, will remain as a feature request for future improvements.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It might be closed if no further activity occurs. Thank you for your contributions.

[chrean]

This is a nice-to-have, but probably not top priority, moving to our Icebox for now, we will reassess in a few weeks/months.