Port "old" API tests for content views

Make module `test_contentview_v2` accomplish most of what module
`test_contentview` formerly accomplished. Module `test_contentview` is still in
place:

* Not all tests in that module have been implemented, so they are untouched.
* Some tests in this module do not test content views, so they should be ported
  into somewhere other than `test_contentview_v2`.
> Do we want to comment that `label` here would fail because it passes data that is not valid for labels,

Yes, an explanation would be good. Also, aren't labels supposed to be immutable?