Write tests for separating messages after a short time gap

Our initial implementation of #1773, from #1815, will leave out the tests, in the interest of getting the new behavior out in today's release rather than the next release in a couple of weeks.

This is a central and tricky part of our code, though: the message-list view-model, MessageListView. So we should follow up promptly with tests for it. Details in the #1815 thread.
