I'm going to argue that the research that says it does not increase testosterone is debatable. Some studies have got positive results, some get negative. I have not read a study that's the be-all and end-all of tribulus testing, although there might be some recent ones I haven't read. It's extremely difficult to get a good standardized extract; source and quality of the herb are important factors. My experience with high quality tribulus is that it increases more than just libido, as stated. If it looks like T, smells like T and also tastes like T, its reasonable to assume that T is increased, although not proof positive.
That said, if your testosterone is already fairly good, then you probably won't notice much in the way of effects. The change is most noticeable if you start off low. Also, it won't increase your T (if it does) to a level that will significantly improve your training.
Now, if you had no idea what it was going to do, why did you even bother buying it?