KIAWAH ISLAND, S.C.—The short jab with his left fist to celebrate birdies and even a few pars. A thumbs-up to the gallery. Phil Mickelson at times looked to be about the only one having fun Friday in a PGA Championship that has become the ultimate test without being extreme. But then, what’s not to enjoy? The 50-year-old Mickelson looked like the Mickelson of old on another windswept grind around the Ocean Course at Kiawah Island, running off five birdies over his last eight holes, the last one giving him a 3-under 69 and a share of the lead. “To know I’m playing well heading into the weekend, to be in contention, to have a good opportunity, I’m having a blast,” Mickelson said. Mickelson shared the lead with Louis Oosthuizen, the South African with the sweet swing and hard luck in getting that second major. Oosthuizen didn’t make a bogey until …