Not that I've seen. Do an inquiry for "horsemeat" and "testosterone" on Google Scholar or any database for academic papers (Lexis Nexis, etc.). I doubt you'll find anything linking the two. If there were actually a link, my guess is that we would have heard a lot more about it in the media by now in light of the horsemeat scandal.
It's important not to perpetuate these rumors because people who don't bother doing their due diligence might see your statement and believe it to be true. I wouldn't be surprised if a major segment of the people who are outraged by the horsemeat scandal honestly believe that horsemeat is filled with testosterone, even if they can't actually back up the source of their belief. It's the stigma (correct or otherwise) associated with horsemeat that is turning this scandal into something bigger than it would otherwise be if the meat were, say, bison meat or pork.