More tests

This PR mostly fleshes out tests, with the one exception of an added CDStar.GET method. I'm not sure, how the API is intended to "feel", but I was looking for a way to get all info about an archive for which I already have the ID - so looking up the docs at https://cdstar.gwdg.de/docs/dev/#getArchiveInfo I knew how to do this using CDStar._rest, but didn't want to call an internal method.