Allows running socket_afinet regression tests in parallel

[olivier]

Summary

They all are using the same port for binding server, so conflicting when running in parallel.

Test Plan

Before:
Set parallelism=8 in /etc/kyua/kyua.conf, and run it.
It will fails once reach socket_afinet_stream_reconnect tests

After:
No more failure

[markj]

Ok, but this is still fragile. It assumes that nothing else in the entire test suite will bind to ports 6667-6669. Probably these tests should specify sin.sin_port = 0 and then use getsockname() to figure out which port was assigned.

[olivier]

Replacing hardcoded port number with dynamic ones.

[markj]

Thanks!