piuparts.py: Move unittests to the tests/ directory

* Move the code for the is_broken_symlink unittest out of piuparts.py into a
  own file in the tests/ directory. Now nosetests can run this test and detect
  the coverage.
piuparts.py: Add unittest for DefaultsFactory / Defaults

* Use pymox to mock the piuparts.guess_flavor() function as it would run
  lsb_release in a subprocess.
* Add python-mox3 to Build-Depends as it is now needed for runnig unittests in
  the package build.
unittests: Add tests/README

* Add a little documentation on naming the test classes and functions.
d/changelog: Document changes