[#3005] Add tests for follow group API

Also refactored some test methods to remove code duplication between
user, dataset and group follower tests.

I started getting DetachedInstanceErrors from SQLAlchemy. It happens
because the tests access the model directly and retain references to
model objects. No idea why it started happening now but wasn't happening
before. I've added an AttributeDict hack at the top of the file to
remove this without introducing too much noise in the diff. Will remove
the hack and fix it properly in a separate commit.