A Queens DA took the stand yesterday, to testify about a detective who fired at Sean Bell, an unarmed man killed in a barrage of police gunfire hours before his wedding. DA Michelle Cort, who took Detective Marc Cooper's statement after the November 25, 2006 shooting, said, "He told us he fired a single shot. He was certain he fired one time." Cooper, (pictured) who actually fired four times and is charged with reckless endangerment,......