Have sanity-bot kvetch if it sees no tests/ change when commit claims to "fix bug"

Modulo some fuzzy-matching way to recognise when a commit message claims to be fixing a bug, it would make sense to check for at least some change under a tests/ directory - if there's none, then no autotest has been added for the allegedly fixed bug. We can then, at least, help reviewers to notice that something is missing (checking what's not there being harder than checking what is) by having sanity-bot kvetch about the commit message's claim to be fixing a bug.

We can expect some false positives ("clean up in preparation for fixing bug", etc.), so the kvetching shouldn't be a -1, but a Hint would be justified, at least.