Increase CI test coverage

1. Run `./contrib/coverage.bash` and look at the coverage report it generates.
2. Write test cases for any lines of code that were not executed.
- Some lines are okay to go without testing -- "verbose" output messages, wildly rare errors like out-of-memory or syscall failure, etc.
4. If those new test cases uncover any bugs, file an issue and/or try to fix them.

Leave this issue open even if you do add *some* more tests. It's an open-ended task, at least until the unlikely day when coverage.bash reports 100%.