Code review comment for ~xavpaice/charm-grafana:bug/1873105

Unit tests fixed, plus one or to minor lints.

"ii. I still don't see any upgrade-charm-related tests. I haven't reviewed the sources about whether the bug in question exists in the current version, but this could be considered a gap."

Testing for upgrade-charm is way out of scope for this change, we don't run that test for a single one of the LMA charm stack right now - though it would be an excellent addition and make life a heck of a lot easier during the release process. I don't think we can reasonably block this change because of something so far out of scope.