Add guidelines around test mocking

[Gudahtt]

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

[exalate-issue-sync]

Elliot Winkler commented: Everyone is going to have an opinion about this, so I don’t know how successful at we will be in getting something merged into contributor-docs. But I still think it’s worth opening a discussion. I see teams misuse mocking a lot (and there are plenty of examples in {{create-release-branch}} and {{auto-changelog}} where we’ve overused mocking).