Fixes unittest.

We decided to change the bahaviour of dispersy_yield_candidates to
return 'unverified' candidates, i.e. all candidates that are either
'walk', 'stumble', or 'intro'.  The unittest did not yet reflect this
change.
Rewrote check to no longer use is_active().
nosetests behaviour keeps me confuddled.

Apparently the loggers (when no other loggers are enabled using -l)
are all enabled by default.  Hence, testing if it is enabled is not a
viable option.

Nosetests is designed for automated tests only, i.e. no parameters can
be passed to change the test behaviour.  As a work-around we are now
using an environmental variable to enable specific tests.
Needed a slightly more specific check.