Test fails if port 1234 is in use on the test host

[mtbentley]

I had znc set up to listen on port 1234, causing test_run_docker_container_with_user_specified_port to fail with The chosen port is already in use!.

I don't know if this should be fixed (by looking for an unused port first) or documented somewhere.

[Rob-Johnson]

yeah, the test can be made more resilient by looking for a free port, rather than hardcoding one.