Add guidelines around test mocking

We should add guidance on how to use mocks when testing.

Some ideas for inspiration:
* https://www.youtube.com/watch?v=Af4M8GMoxi4
* https://martinfowler.com/articles/mocksArentStubs.html

Start by reviewing the proposed references, including Martin Fowler's article on mocks and stubs and the linked YouTube talk, then inspect the contributor-docs structure to find the appropriate testing guidance location. Done means the documentation clearly explains how to use mocks when testing and incorporates relevant guidance from the references.

Written by the indexing model from the issue text.