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 
Do you know of any files in freeseer that currently depend on a more significant software structure like a database, mtomwing? I can try to make a more complex example.

When I was reading the documentation on pytest, being able to store the result of a function call seemed very handy. For example, if you needed to test several objects created from a constructor with different parameters through out the unit test file, then the fixtures could store a variety of different object and the developer writing the tests would not have to recreate the mock objects each time.