> The way you test code cannot (and should not) be decoupled by the way in which you architect the code itself.

I'm not sure I fully understand. In my view, it's pretty evident that tests should test behavior and not structure. Coupling to implementation details is unavoidable but should be managed, and ideally minimized, to keep the test suite robust and maintainable.

Given the choice between a test suite that monkey patches out some dependency (redis, say) on a per-test basis, and one which substitutes a fake for this dependency in some centralized composition root, which one do you prefer, and why?