Hi Miljenko,
I checked the examples you provided. Issue with persistent test is that you reuse environment object for both sessions so once you close the first one it will invalidate the environment so the second one can't operate on it any more. If you create two separate instance of the environment that will be dedicated to the session your test will work just fine and in my opinion that is exactly same behavior as for the non persistent state.
Regarding the topic in general, I think it is the right approach that you can signal processes regardless of its origin session. That is due to processes can be long running (days, weeks, etc) so once you loose session handle they should be still reachable in some ways.
But I see your point too, to be able to signal only processes that origin from a particular session. This can be done by providing process instance id to the signal method, as far as I know. Probably not the perfect way of doing it but should work. This could be a subject for more detailed discussion to try to find out if scoping signaling to only one session is of interest to the community.
HTH