North Korea on Friday credited the country’s new leader, Kim Jong-un , with spearheading past nuclear testing, adding to official efforts to portray the young son of the late leader Kim Jong-il as a confident military commander. The official Uriminzokkiri Web site said Friday that Mr. Kim had “frightened” the country’s enemies by commanding nuclear tests. North Korea tested nuclear devices in 2006 and 2009, but the Web site did not specify which tests Mr. Kim oversaw.