LOUISVILLE, Ky. — Hall of Fame trainer Bob Baffert said Sunday that his barn has been told Kentucky Derby winner Medina Spirit failed a postrace drug test, the latest doping scandal for horse racing and arguably the sport’s premier trainer. Flanked by his attorney Craig Robertson in a morning news conference at Churchill Downs on Sunday, Baffert said Medina Spirit was found to have 21 picograms of the steroid betamethasone, double the legal threshold in Kentucky racing, in a postrace sample.