Add the ability to test the order flow

[franckc]

Before their store goes live, it is natural (and recommended) for a merchant to test it by placing a few test orders. We should make it easier for them to do so. Currently their only option is to place a real order.

A few avenues to explore:

• Shop configuration: clearly separate out production vs testing settings
• Payment side
  • Support test credit card and paypal payments (with dev API keys)
  • Support test crypto payments on Rinkeby
• Fulfillment side
  • Support test Printful orders. TBD does printful have a sandbox API?