This is false. I'll explain how articles such as these may get that idea, but it's only a theory without any proof to support it.

The bull was a fairly prominent symbol in Greek history, seen time and time again. There's evidence of this as early as 2000 BC; wall paintings of leaping bulls were found in Knossos. Bull sculptures were also found; this may very well have been some sort of ritualistic practice.

The most well known ancient Greek reference to the bull is the myth of Theseus and the Minotaur. I'll summarize the beginning of the myth which is the most important part in this context. King Minos promised to sacrifice a beautiful white bull to Poseidon but switched it for a normal bull because he wanted to keep the nicer one. Furious, Poseidon unleashed the bull to rampage through Crete. Further, he made King Minos' wife fall in love with this bull; they had what I assume was extremely awkward sex and the Minotaur was born.

In the context of this myth, the bull is a symbol of strength; it's meant to destroy Crete. It's also a symbol of male fertility because of what happened between the bull and Minos' wife. This male prowess would certainly come in handy at a sporting event such as the Olympics, so I can at least see where some people would get the idea that ancient Greeks would use bull testicles as a steroid. But there's absolutely no direct proof to support this.

Source: "The Greeks: History, Culture, and Society" by Ian Morris