Task level parallelism is unsupported and undocumented feature that can easily cause unreproducible build failures unless you are very careful. There is a hidden flag to enable it, but you shouldn’t use it. I think they are going to bring it for real when Gradle transitions to the new configuration model.
What is your issue with having each test task execute the actual tests in parallel (i.e. why is maxParallelForks not sufficient for what you need)?