Suge Knight Won't Testify Against 'Keefe D,' Says Tupac Shooter Isn't Who Cops Think It Is

Suge Knight is surprised there's finally an arrest in the murder of Tupac Shakur, and even though he was an eyewitness in the case ... he says he'll refuse if called to testify against the suspect, Duane "Keefe D" Davis. TMZ spoke exclusively with…