We should introduce unittests using e.g. the built-in unittest package. The CLI (when finished) will also provide a way to run those unittests (see #57). I think we should package those testing features into a package named test, shouldn't we?
1365b10 introduced the test module. For now, this should be enaugh. If we have more checks and test cases, we might want to move the tests into a new package.
