add tests

[ndeet]

Challenge: the other commerce payment plugins use the api credentials of the payment providers sandbox urls. As BTCPay is self hosted there is no such thing.

Option 1: mock the responses and payment providers
Option 2: write tests against a public test BTCPay instance, maybe the official one? (Needs to work with existing paired keys or allow to automate that somehow)
Opition 3: leave out those tests :(