After discussing this with sanderbangma and stronk7 it was decided that we need to add test coverage for Airnotifier.

Quoting Eloy:
_ideal: we should have message/output/airnotifier completely covered. Say mocking the airnotifier service, using a server somewhere @ CI infrastructure or allowing, in some way, to be able to inspect the stuff sent to it (we already have some "echo" services @ download.moodle.org/unittest providing that sort of facilities). We just need an airnotifier "echo" @ https://github.com/moodlehq/moodle-exttests_