Uh, where is the proof of this?? It's kind of hard to conceal a test of a nuclear weapon. It tends to leave unmistakable traces behind. Since our troops have been pretty much all over Afghanistan chasing the Taliban, I suspect they would have found said evidence, if it existed.
Personally I agree and don't believe AQ has suitcase nukes. Those things are meant to be very high maintenance and hard to keep up. However, if the US had found evidence that AQ had tested them and might well have more, the last thing they'd do i sannounce it publicly. That would give AQ instant street cred, throw jitters through the market and public and make a lot of people start getting nervous about the war...behind the scenes feeler to friendly governments and redoubled efforts by Intel would be their key course of action.
That said, I think the story is rubbish.