If you think that design is in test code then you have my pity. I suppose you have never seen a design document; people are too lazy for that. I do write documents. I write requirements and functional specs; I don’t write implementation specs much anymore unless it’s post-development because they are too volatile.
I wrote in Smalltalk for a while, I never heard of TDD; Beck’s book came out in 2000 and Smalltalk was a fading memory by then.
You’re the second one who has told me that tests are for interface design. That is completely wrong, factually and conceptually.