NEW YORK — Jam Master Jay’s best friend told a Brooklyn jury he grabbed the Run-DMC star’s gun moments after his killing, then ran outside in a desperate, failed attempt to chase his killers. Randy Allen, who knew the hip-hop icon since middle school, testified Tuesday he was sitting in the control room of the DJ’s Queens music studio on Oct. 30, 2002, when a pair of shots rang out. He didn’t see the shooters, but he ran out when he heard the noise and saw the bloody aftermath, he said. “I look down, and Jay is laying there, shot. He’s got blood coming from his head. … I grabbed the gun that w…