Code review comment for ~ilasc/launchpad:bug-1818755

I suggested earlier that some unit tests for SnappyDistroSeriesVocabulary might be a good idea. I now think that's definitely needed: there are enough special cases in here that it doesn't make sense to try to test for them only in passing in browser-level tests.

Aside from that, I think we're now converging on something that will work, but I've left a number of detailed comments. Thanks.