Actually everything is testable, but ... not everything is easily testable - even if it should actually be.
Play did a great job, offering tools to make tests easily.
But testing a publish-subscribe architecture has some pitfalls we have to be aware of.
Like in a request-reply model, the system under test is the web app, which lives in a dummy server (a server specifically designed to be instantiated in a test) and interacts with a dummy client.
The system under test must be treated as a black box, and this is just what happens when, in a request-reply model test, the client sends a request and checks the expected reply.
In a publish-subscribe model however you must have some sort of control on what and when is published, in order to verify what and when the client is supposed to receive.
This must be designed carefully, otherwise the risk is to test a system that actually is not any more the system it ought to be.