test_cv_subscribe_system

Lets have another round and hopefuly prevent another bazzillion comments :-) :

1) if you thing that test_cv_subscribe_system attempts to test too much at once and I should first port the old promote/publish api tests before this one, tell me as well.

But I'd like to have this thing merged and then to move on to the rest of content-view stuff :-)
Would be best to continue returning a response object. As the caller can inspect other response attrs if necessary?