The PageviewAPI feeds of the analytics pipeline in prod but what clients see is an isolated http entry point. I think we could deploy a mock version of it to beta. I have worked in several SOA testing environments in which a similar approach was used and that was of value.
On our end we know we could benefit from a testing stack for pageview API better than the one we have.
[using live enwiki data in an extension on the beta cluster] is fine for testing the extension but not useful for testing the API (ie. detecting that a change to the Pageview API code would break the extension) so it would be nice to get a mock service eventually. (Also the extension now has an awkward smoke test which requests data from enwiki and tries to guess whether that data is right; a deterministic mock API where you can fix the expected return value in tests would be helpful.)