The push to bring self-driving cars to American roads got a significant boost on Thursday when the nation’s chief auto safety regulator essentially cleared Tesla Motors Inc.’s Autopilot system of fault in a fatal 2016 crash. The U.S. National Highway Traffic Safety Administration found that the owner of a Tesla Motors Inc. Model S sedan that drove itself into the side of a truck in May had ignored the manufacturer’s warnings to maintain control even while using the driver-assist function.