E2E test for pre-authorisation flows

[gcatanese]

As pointed out in #52 we don't have E2E tests that validate the possible flows.

Extend the E2E testing for the subscription use case to cover the following:

• Verify that the admin view is visible after sending the webhook containing the merchantReference (AUTHORISATION eventcode)

• Add adjustment test

• Mock the AUTHORISATION_ADJUSTMENT webhook and verify that the admin view has changed
  - [x] Add extension test (it's similar to adjustment, unless we want to test this specifically, I'd leave this one out)

• Add capture test (verify that the next screen is green)

• Mock CAPTURE webhook

• Add reversal test (verify that the next screen is green)

• Mock the CANCEL_OR_REFUND webhook

• Mock CAPTURE_FAILED (or CAPTURE, success=false) webhook and verify that we have a reversed item in the admin view