Add mention in CONTRIBUTING.md that MR should have a test plan

What does this MR do?

Often, in projects, I want to review the work that has been done by the team. I end up asking questions in Slack to the developers, asking them to tell me what I should check to make sure the change is effective.

MR should have a test plan so anyone, even not familiar with the MR or the codebase, can jump in and check that the change addresses the problem/feature/fix. Even if the change is trivial, it should be a no-brainer to know how to test the changes brought by the MR.

We have unit tests, but those tests are for the code itself. Not for a human to test the changes by hand.

This might also help to not forget stuff that should be tested, and help during the code review process. It's like TDD for MR.

Are there points in the code the reviewer needs to double check?

Is CONTRIBUTING.md the right place to put this note?