I'm truly curious what those tests would be expected to show.
If Zimmerman shot Martin while Martin was on top pounding him, then you would expect GSR on Martin and Martin's blood on Zimmerman. If they are not present, it would call Zimmerman's account in question.
Yes, I understand that. But it isn't Zimmerman's account that we're questioning here - as far as I know, Zimmerman's statement has not been released. What we do have is several eyewitness accounts of Martin on top of Zimmerman immediately before the gunshot. We also have an audio recording of somebody (at least one witness says Zimmerman) yelling for help right up until the shot.
I'm not saying they should not do the tests - I'm just not sure how useful the results will be, because we already know they were fighting before the shot. We don't know who started the fight, and I don't think those tests will tell us that.
I am also not convinced these tests have not been performed. The police report that they still have Zimmerman's clothes from that night, and his gun. I've seen no reports that his or Martin's clothes have not been tested.