Recently I saw a question on StackOverflow about Dialogflow ES testing framework.
Dialogflow CX does have built-in Test Cases. That is one more reason why CX is more powerful and better suited for building complex flows than ES.
However, there is no standard framework for ES.
If you want to do conversation testing, my suggestion is to just use the pytest library and do it inside the PyCharm IDE (which has a free community edition).
In this article I will talk about a bunch of things which you should consider when creating test cases for Dialogflow ES.