Increase test coverage on backend

[spwoodcock]

Is your feature request related to a problem? Please describe.

• We have some good backend tests for endpoints.
• They are invaluable for checking changes to the backend.
• It allows for fast changes as we have more assurances our code didn't break anything downstream.

Describe the solution you'd like

• More test cases for the backend APIs.
• Functionality is reasonably stable now, so we should add tests for most.
• Coverage of about 80% with good edge case coverage would be great going forward.
• This is key to avoid breaking changes when we add new features or modify existing code.
• The response for each endpoint should be inspected and checked for the essential fields it should have, plus to check if they are valid values.