Make it easy to test apps

[jedateach]

It is tricky to write unit tests for apps that use the omnipay module. You currently need to replicate much of the setup used in the omnipay tests.

Currently difficult:

• Making use of mocking
• Getting the correct PaymentGatewayController endpoint to call for mimicking off-site redirects & callbacks.

This needs to be documented.

It may be worth making this improvement at the same time as upgrading to omnipay v2 #23

Also, worth documenting is how to mimic another running session, by creating a new TestSession during the test.

[jedateach]

Related #71