By the same people.
Yes, but they were all conducted over a period of decades, either underground, at sea or in barren deserts, at ground level. And very few thermonuclear weapons were tested, it was mostly the fission triggers being QA'd. I'm sure you know the nuclear winter scenario is blamed on the smoke from fires, not on the results of the nukes themselves. In a real nuclear war airbursts would be extensively employed to start fires over wide areas.
I'll concede that we have no way of knowing if nuclear winter is possible until we test it in the real world, and perhaps it wouldn't happen if we did, but your argument is bogus.