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 have created, https://gist.github.com/wigglier/341ebb33baaa57ee4a57 based on the framework/test_presentation.py file as a potential example of using the fixture to hold something costly like a db connector. However, the fixture does not behave as I expected. It gets recreated in each of the three top level tests in the module. Will debug it later this weekend hopefully.