During 0.9 support was added for TestListeners. These can listen to test execution events and are intended to be test framework agnostic. However, TestNG support was not finished because it would require that Gradle have a compile-time dependency on TestNG. This causes Gradle to be restricted to that version of TestNG for projects, and we did not want the restriction.
We need to figure out some way to compile against TestNG without limiting what version of TestNG a project can use for running it's tests. (There is a similar issue for JUnit, but Gradle has long had a compile-time dependence on it).