Ignore PEP8's E501 (lines >79 chars) in tests

According to [naming test methods](https://ckan.readthedocs.org/en/latest/contributing/testing.html#naming-test-methods) in our testing coding standards, we should make the test names clear "...even if it means your method name gets really long, since we don’t write code that calls our test methods there’s no advantage to having short test method names". Given this, we shouldn't validate E501 on our test suite.