Add a docstring to hacking unit tests

This commit adds a docstring to the hacking check unit tests
explaining how the tests work. To someone who isn't well versed with
how pep8/flake8 works the tests cases are a bit of black magic.

Change-Id: I5dc61612a5fec42443d8d89eaf700bb17b4b0842
Merge "Add a docstring to hacking unit tests"