Increase test coverage

While at the PyCon 2023 sprints I contributed to beeware/briefcase. One interesting thing I noted is that they have 100% test coverage. While 100% test coverage doesn't mean your tests are *good*, it did have the effect of forcing me to think about un-exercised code paths and write an extra test for a path I was missing. (The CI wouldn't pass with anything less than 100%)

That motivation was useful, and my contribution was better for it. I think it might be worth taking some time to increase our coverage, see how close to 100% we can get.