Improve test coverage of marbles \times unittest

[thejunglejane]

marbles itself is fairly comprehensively tested, in terms of lines and also features, but we don't very well test how marbles interacts with all of the features of unittest. For example, some of the assertions we're wrapping in marbles may behave strangely depending on how we've wrapped them (we've seen problems before where things like assertMultiLineEqual don't faithfully pass the msg argument to all assertions they then make, which broke our annotations, but only when that method got called with something that wasn't a str).

We should try to test marbles against all assertions in unittest, possibly by running unittest's own test harnesses. We may also want to test interactions between marbles and other features of unittest, but let's start there.