server: Write test for spawning subprocesses in workers

[achimnol]

Spawning additional subprocesses in the child workers when use_threading=True because only the main thread can receive signals from child processes. This line instantiates a child watcher before launching workers, as described in the Python docs.

Currently there is no explicit test case that tests if this actually works. Let's add one.

[achimnol]

Now this is no longer a significant problem as asyncio has stable defaults (ThreadedChildWatcher and PidfdChildWatcher).