Hopefully, you've had the opportunity to look over Darren Wilson's grand jury testimony in the case of the State of Missouri versus Darren Wilson. The "versus" part is a complete joke, of course. If you read through the testimony and listen to the statements of prosecutor Bob McCulloch, you'll see that it's more like "Missouri and Darren Wilson versus Michael Brown." Wilson was not put on trial, the dead black teen was, and Brown didn't get to testify.
If Wilson had been brought to trial, his testimony would have been subjected to cross-examination. It's possible that Wilson wouldn't even have taken the stand in his own defense — many criminal defendants do not — because he wouldn't have wanted to face cross-examination.