Resolve consistent failures related to "address in use" etc

[consideRatio]

From #425 (comment)

address already in use failures

This one's tricky. Getting rid of the hard-coded port (e.g. NOT_54321_PORT = get_unused_port()) would for sure get rid of this, but it might be indicative of a deeper cleanup problem... perhaps the atexitasync stuff isn't firing properly, or some other async shenanigans? Perhaps it's not interacting correctly with pytest's freaky stuff?

[consideRatio]

I've failed to resolve this, stuck failing to setup tests on my local computer =/