Add documentation to how our testing framework is setup and how to write tests

[ckbedwell]

Problem

We have quite a comprehensive testing framework that does a lot of the heavy lifting for making writing tests as pain free as possible however none of this is documented.

Completion criteria

Please create documentation in the repo that outlines:

• How our testing framework is set up and initialised
• What assumptions it makes and does for you out of the box (e.g. the custom render method, MSW happy paths)
• What helper functions are available (e.g. runTestAsViewer), options you can provide the custom render method (e.g. routing options, custom meta, etc.)
• How to write tests
• Outlines how larger features have a meta testing framework with their own helpers to reduce writing boilerplate. See PR: Check form redesign #847