A toxicologist testified in New York that, based on autopsy reports, a black man who died in an alleged racist attack in the city's Howard Beach section had taken a near-lethal dose of cocaine no more than two hours earlier. The testimony by Dr. Thomas Manning, chief toxicologist in the Nassau County medical examiner's office, conflicted sharply with that of a prosecution expert witness, who testified that the victim, Michael Griffith, had taken cocaine at least 12 hours before he died. Manning was called to testify by defense lawyers for four white teen-agers charged in Griffith's death, two with murder and two with manslaughter. The teen-agers are accused of killing Griffith, 23, by chasing him onto a highway and into the path of a car Dec. 20 after a melee outside a pizzeria. The defense is seeking to show that Griffith was high on cocaine and acting irrationally when he ran onto the highway.