Not all test methods need docstrings

Our [testing coding standards](http://docs.ckan.org/en/latest/contributing/testing.html) says that:

> Test methods and helper functions should have docstrings.

But if we follow the other advices (having small tests, testing only one thing,
naming well), their intent will be evident from the test name. It can be useful
to have docstrings on some tests and helpers, but not all of them.