Write tests for msglist opening at first unread, and setting

[gnprice]

This is a follow-up to #80. For the launch we'll be shipping an implementation (from #1517) that doesn't yet have tests, in the interest of timeliness. It'll be good to follow up by writing those tests.

The code to be tested can be seen by looking at the one commit in #1517. Before merging that PR, I also intend to edit it to add TODO comments, pointing at this issue, in the relevant areas of our test files.