I forgot if we discussed this before, but I'm slightly uncertain about the naming in results_.
Before, we used mean_validation_score, now it's test_mean_score. I'm wondering if it should be test or validation. I would call this the mean cross-validation score (I guess I named the previous named tuple).

After all, it's called cross-validation, not cross-testing ;)

ping @jnothman @raghavrv @GaelVaroquaux

I don't feel very strongly but I think it deserves some consideration (I just saw me using it in the book and it felt strange).