test/unit/api.py: Fixed a few issues caught by pyflakes

* Removed haas.config import - it was unused.
* Two pep8 violations; space after the comma and two blank lines between
  top-level constructs.
Wrote some tests exposing breakage in group_{add,remove}_user.
Fixed some bogus conditions.

These were causing the breakage caught by the tests in the previous
commit. I'm not sure why someone thought this would test for set
membership.