President Trump's comments about restarting weapons tests are not likely to lead to mushroom-cloud explosions over the New Mexico desert or seismic shaking underground in Nevada, according to the nation's foremost experts. But questions about  what kind of testing Mr. Trump does have in mind have been rippling through the nuclear nonproliferation community. In an interview on " 60 Minutes ," the president elaborated on remarks he made last week announcing a plan to resume U.S. testing of