add tests

This isn't completely straightforward, since we have to mock `discid.read()` and we probably should mock getting data from MusicBrainz (to make offline testing possible and keep the results unchanged).

Additionally, it isn't yet clear to me on what level we should test.
Testing only some internal functions isn't doing much good. The main thing to keep working is the overall workflow.
So the most interesting thing would be to run isrcsubmit with input and output redirected to files.