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 
This paragraph was specific to unittest (which didn't support auto discovery for a long time). Since we're using pytest, this paragraph no longer applies.<sup>1</sup>

I don't think we even need to put tests inside of `src/freeseer/tests/` for pytest to find them, but it does make the codebase more organized by having all tests in one place. So maybe this whole paragraph can be replaced with

> All our tests exist inside `src/freeseer/tests`.

1: http://pytest.org/latest/goodpractises.html#test-discovery