Use automatically allocated ports in the zebrad tests

[teor2345]

Motivation

The zebrad acceptance tests sometimes fail due to port conflicts

Solution

If these failures continue, we should:

• ask the OS to allocate a listener port, by configuring port 0
• read the actual port from zebrad's logs
• launch the test or conflicting process with that port

We won't be able to read the port for some listeners, because the crates we're using don't expose that information.

Priority

This is a low priority, unless we see further test failures.

[teor2345]

We've done this already, and the rest of the tests need fixed ports.