Move the Contribution Testing Guidelines to framework docs

[Swiftaxe]

PR #229 contains contribution guidelines for writing tests. It has been sitting for a while, but according to the author, it is still highly relevant, and the content is correct, but it belongs in the framework repo.

• Open a new PR in the framework repo, and move it there, as-is.
• Close PR feat: Add testing contribution guideline #229 with a note.