FIX fix the removal of set while resolving merge conflicts

COSMIT minor pep8 fixes in sklearn.utils.testing
TST Added partial_fit estimator checks

* Test for estimator restart upon fit
* Validation of classes argument
* Test to check that partial_fit returns reference to self
* Refactor multiple imports from utils.testing into single import
TST Added tests for estimators implementing partial_fit
ENH method_filter will now accept string or list of strings

ENH type_filter will skip if the list is empty
COSMIT minor cosmetic and pep8 fixes