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 
Tests should never use external resources that are outside of our control. The best way to do a test like this would be to mock out the call that `db.add_talks_from_rss(...)` uses to fetch the feed URL and have it return some static data.

Since this is just an example in the docs, you probably don't have to go that far with it right now. Instead I say just nuke it so people don't get the wrong idea.