Improve test coverage

[Luthaf]

As of writing this issue, the test coverage is 53%. We should add more tests to bring this up to at least 80%, 90% would be better.

Tests are in the test directory, and they are Python scripts checking the output for different run of the main binary.

Code coverage is then collected by codecov. If you want to help here, click on the codecov link, pick a file with less than 80% coverage, and add a test checking the output for this specific code path. If you don't know how to trigger a specific code path, ask here. I am willing to mentor anyone wanting to get started on the code with this issue.