Support pagination in Collections API test helper

This changes the Collections API test helper to support pagination. 

It includes the `documents_start` and `documents_total` keys in the response body, so that apps using the test helper can test their pagination behaviour.

It also allows each of these values to be overridden by providing the `return_body_options` key in the `options` argument. This is so that tests can simulate scenarios with multiple pages of results.