Bug 972893 disable all docstrings in nosetests

@AdrianGaudebert r?

The annoying thing is that we need to put it in every test base class that subclasses immediately from `unittest.TestCase`. And we have a lot of them.

As I did some search and replaces, I noticed there are some test classes that inherits from `unittest.TestCase` but none of the tests within use docstrings. Perhaps that's silly to fix. But once we add more tests some day, we'll have the trick already set on the class.