Add testing coverage for Kafka authentication

There is some authentication functions added !1093 (merged) that require additional test coverage: https://git.ligo.org/emfollow/gwcelery/-/blob/064df3f7858dd18d7b8178cbbd088faeed9f3180/gwcelery/kafka/bootsteps.py#L85-104

There are also other sections of kakfa/bootsteps.py that require testing coverage too.