Migrate to pytest
There isn't much nose-specific in the project yet, and it doesn't seem
to be actively developed upstream. See:
https://nose.readthedocs.io/en/latest/#note-to-users
Pytest has many useful features like log capturing, easy parametrizing
of tests, so let's use it instead.
Signed-off-by: Nils Philippsen <nils@redhat.com>