Add Spawner test for the `--unload-on-kill` locking other spawners

[saikishor]

We have observed this during our tests on TALOS. I'm not sure how to add a deterministic test to this as we need to run both at mere same time for this to happen. I can take a look later.

might work with a python integration test, but they are flaky (or at least I haven't managed to configure them properly)

Originally posted by @christophfroehlich in #2559 (comment)