Bug 1062461 — Change default server port for tests.

https://bugzilla.mozilla.org/show_bug.cgi?id=1062461
Even simpler:

http://nodejs.org/docs/latest/api/net.html#net_server_listen_port_host_backlog_callback

set the value to 0, not 54321

then once it's started, in the event callback you can set it to its real value