Ensure service desk related logic has 100% code coverage
gitlab-org/release/retrospectives#4 (closed) describes an incident that involves the service desk feature, specifically it describes how certain methods were unavailable due to how we include modules into classes.
We should make sure that service desk related code has 100% code coverage, including any tests necessary for columns that can contain NULL values (so we test both the NULL and not NULL cases). This won't solve the core problem (= having to include modules using a particular approach), but it will allow us to catch any regressions.