You question is probably off topic for stackoverflow...or a duplicate of Automated testing in RPG (or other ILE languages)
But I'll throw this answer out as a general, yes tools exist; without trying to specify any particular one.
In addition, you shouldn't necessarily be looking for tools for RPG; in particular with ATDD you probably want tools that can test 5250 applications. Yes such an app is probably RPG, but it doesn't have to be and the tool won't care. The point is you could have an RPG app that has a web front end; a different tool could be usable there.
Lastly, let me say that the tools out there are going to be commercial tools for the most part. And they aren't going to be cheap.