increases test coverage on teuthology.worker

This increases the test coverage on teuthology.worker from 0% to 50%.  The goal was to write tests, without modifying teuthology.worker.  We could definitely get more coverage with better tests if/when we're ready to refactor worker.  Until then, 50% is still a great improvement on the existing code.

Signed-off-by: Andrew Schoen <aschoen@redhat.com>