Code review comment for lp:~jpds/launchpad/fix_489363

We went through a lot of back-and-forth on IRC, but now I think it's in fine shape.

The main thing I fought for was to inject a fake timestamp into the test, so that it has an exact, predictable text to match for. A separate test then verifies that this isn't hiding a failure in composing the human-readable text for the log message.

I understand LoggingMixin now has the appropriate docstrings. With that, r=me.

Jeroen