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 
I think the above paragraph (i.e. "If you have not already installed ...") isn't needed because in this paragraph we say that no additional configuration is needed if you installed the development requirements. But this paragraph can be improved by having "installing the development requirements" reference the PyPi packages section (like you did in the above paragraph).