acceptance tests for this service

[Jumpy-Squirrel]

Similar to the acceptance tests in reg-attendee-service, this service should have automated given/when/then tests for its endpoints.

The tests should cover each endpoint with

• success case
• bad request cases
• security tests (if no token is sent, cannot do anything, if not admin cannot do anything, ...)

All tests should live under /test/acceptance, with mocks under /test/mocks so code coverage is correct when collected over /internal.

It is probably easiest to start with all the negative cases.

A recorder mock should be needed for /internal/service/mailsrv only. Just like in the attendee service, the tests can use the inmemory db, load a config file under /test/resources, etc.