Update the references in contributor testing documentation to refer to pytest instead of unittest.

The testing documentation page currently shows examples of using the unittest framework to create and write tests. However,  Freeseer is moving towards pytest for unit testing. Therefore, the documentation page should be updated to refer to pytest instead of unittest. For example, python examples that show how to extend the existing test set with the unittest module should be updated to reflect the pytest module.

related #420 
related #484 
fix #566 
We should only use links like this for external content. Since you're linking to a section within the docs, it's better to use a reference. See http://sphinx-doc.org/markup/inline.html#role-ref (at the bottom of that section it explains why you should prefer it over links).