Add tests that check generated output

[HenningTimm]

The current tests only check if the result returned via a --check call is right. In other words, they only assure that valid files pass (with error code 0) and invalid files fail (with error code 1).

To make sure, that tsv files are generated correctly, we need to prepare some expected tsv output files, call the tool in the test, and compare outputs. This can either be done via tempfiles or by adding the option to write to stdout.