Test coverage [tracking ticket]

[crazyscot]

At the time of writing test coverage is pretty spotty. Over time I plan to improve this.
However it doesn't make sense to pour testing effort into parts of the code that are intended to change significantly.

This will be a piecemeal effort, in parallel with feature development.
Some parts of the code will likely require refactor to be more unit test-friendly.

• Exclude test code from coverage (integration tests are already excluded; cargo-llvm-cov supports marking modules as excluded, with the nightly toolchain)