Add test to verify renders don't add response to Request

We already added a test that verified that a renderer no longer
mutated request.response, now we also have  a test that verifies that if
no response exists, then it won't be touched either.
Remove useless function that was used only for Chameleon
Remove _registerRenderer function from tests

It is no longer used, and thus wasn't covered.
Add pragma: nocover due to Py3x/Py2x differences
Remove un-used Dummies: DummyFactory/DummyRendererInfo