Most tests have been performed underground. And for an above-ground test to cause anything close to a nuclear winter, you need a lot more material than the bomb itself and the soil beneath it. Tons and tons of debris would need to be shot up into the atmosphere.

So, the theory of nuclear winter depends on bombs detonating over cities and forests, causing massive amounts of debris, and also inducing firestorms that would act as chimneys for even more smoke and particles to be funneled up into the troposphere and stratosphere.