Add automated test cases and coverage reporting

[Rangi42]

RGBDS's contrib/coverage.bash script is a model here: it builds the project, runs a bunch of included test cases, and (if the tests didn't fail) reports on code coverage. Editing libplum is even more finicky (if I refactor something, am I really sure it hasn't broken some edge-case image?) so an in-repo test suite would be really convenient.