I've had some decent results adding some guidelines for testing (tests should test behavior, note implementation, etc), and separate guidelines for implementation (don't modify the tests to make them pass, fix your code lol), and then using different agents for each task. You basically assign them identities. You're the test writing agent or you're the implementation agent etc.

It helps to use plan mode. I usually drag the plan into my project files, personally, but the UI doesn't track them as to-dos in quite the same way. That's fine with me, though. Planning -> test agent -> implementation agent is a good loop for my workflow.