Add support for the comment field in upstream tests.

Handles the change from json-schema-org/JSON-Schema-Test-Suite#357.

Right now nothing is done with the comment (i.e. it's not shown
during test failures). Maybe later that'll change.

Author: Julian

jsonschema/tests/_suite.py

@@ -169,6 +169,8 @@ class _Test(object):
 
     _remotes = attr.ib()
 
+    comment = attr.ib(default=None)
+
     def __repr__(self):  # pragma: no cover
         return "<Test {}>".format(self.fully_qualified_name)