Use OS-picked ports in the cli.rs tests

[RNabel]

Starts #37 by using dynamically picked ports in the cli tests.

This PR:

1. Let's the OS pick a free port by starting up a temporary TCP server against port 0 and checking which port it took.
2. Creates a shim around the Child + port to avoid passing around (Child, port) tuples.

This way of getting a port isn't 100% reliable but the alternatives of e.g. having the node print its IP/port to stdout and then parsing it felt worse. Happy to make changes if you have other ideas!