Add some more tests

[markuspf]

[wilfwilson]

We have 100% code coverage now. I think we should try to maintain that as far as possible (this is encouraged by the use of the Codecov PR status checks, which fail is the diff is not 100% covered).

I also agree it would be nice to expand the tests so that we are testing a bigger variety of things, and also to add some more difficult tests that perhaps take a while (to act as an incentive to make things go faster).