When I removed some of these tests (initially I copied all, because I didn't really pay attention to this), I considered ways that the multiprocess version might go wrong. But because I considered also the possibility of _another implementation_ of multiprocessing (other than the one that I implemented in this PR), I couldn't convince myself to remove some of the tests. Otherwise I would have removed them, since I also like a short and sweet set of tests. One of the biggest factor for me to include these tests is that the tests are very lightweight, so I think that it should be ok to include a bit more.

In this case, since my implementation of multiprocessing changes how the vocabulary is produced, and also by the very nature of multiprocessing, the "max_df" and "min_df" should be properly considered (those require the knowledge of the whole corpora), that's why I included those tests that test the vocabulary and "max_df" or "min_df", which, unfortunately, is almost everything.

One test that is possible to be removed is the grid search test. As mblondel commented, it might not be necessary to include the grid search. But ogrisel also pointed out that integrity testing is important also. That's why I haven't removed that test until now.

I hope this clears things out.