fix subclassing with `_suites = [foo]`
Merge remote-tracking branch 'yelp/master' into multiple_suites

Conflicts:
	test/test_case_test.py
	testify/test_case.py
simplify the union of cls and bases test suites.
Merge pull request #201 from bukzor/multiple_suites

fix subclassing with `_suites = [foo]`