Add tests to ensure callbacks do not realize data

Currently, we do not have tests that ensure the data is not realized. If we ensure the data is lazy this will help with memory.

Currently our test data is small so it is realised by iris with a simple load. Therefore, we need to consider the data we are using as well.

Locate the callback-related tests and inspect how their current test data is loaded. Use data large enough that Iris cannot realize it through a simple load, then add checks that callbacks preserve laziness and do not realize the data; the tests are done when they would fail if realization occurred.

Written by the indexing model from the issue text.