Increase CI test coverage

[Rangi42]

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.
3. 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%.

[Rangi42]

We now run LCOV automatically in CI! And lines that don't really need testing are excluded from coverage (e.g. fatal I/O errors due to disk failure). Also LCOV 2.0 has a dark theme. :3

[Rangi42]

RGBASM has reached 100% coverage (for include/asm/ and src/asm/)! Three more programs to go.

[Rangi42]

RGBFIX (src/fix/) has also reached 100% coverage!