KABUL — The Taliban captured a key district center in Afghanistan’s southern Helmand province on Thursday while in the country’s north, an officer turned his rifle on sleeping colleagues, killing nine policemen, officials said. The fall of Sangin district, once considered the deadliest battlefield for British and U.S. troops in Afghanistan, comes amid the insurgents’ year-long push to expand their footprint in the Taliban heartland of Helmand. Since the withdrawal of foreign NATO combat troops from Afghanistan at the end of 2014, and with only a smaller, U.S.-led advise-and-training mission left behind, Sangin has been seen as a major test of whether Afghan security forces can hold off advancing Taliban fighters.