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 will look for a database unit test.

For the example, if an object you can create has several corner cases and you need to test the object corner cases in multiple scenarios. E.g. 4 functions can take the corner case objects as parameters. Then instead of recreating the corner case object for each test, you can create them as fixtures and pass those into the tests.

Although I might not have a complete understanding of fixture objects.