Add unittests

it should be easy for the remotes, unclear how easy it will be for the scripts, still there has to be something to test code changes, even better if done before the py3k porting

sadly it has not been done before the py3k port started, but i've already have some tests (from other contributors) and ported them to pytest

i'm going to expand them as i continue the port (and at the same time i'm also using them to verity bts-link works on stable, using a docker image)