Re-evaluate test coverage

[setrofim]

At the moment, we have a number of subpackages that are being excluded from coverage checks (see IGNORE_COVERAGE in the top-level Makefile). This is generally for good reason, and the code in those packages is being exerecised elsewhere (either in other subpackages' unit tests, or via integration tests).

However, we should go through these one-by-one and make sure that there aren't units tests worth adding.