Refer to the PoC https://github.com/evacchi/reactive-messaging-poc and realize a testing utility to produce events on-demand for testing purposes.
The PoC provides a generator that emits 1 event per second. You may use whatever your reactive streams implementation of choice provides for a subscribable sink.

Because this is for testing purposes, we can choose whatever implementation of Reactive messaging we want. I suggest Mutiny with a UniCast or BroadCast processor:
https://smallrye.io/smallrye-mutiny/#_how_can_i_use_a_unicast_processor