netinet tests: Add test for IPv6 mapped-v4 bind problem

Test fix in 637f317c6d9c, verifying that when ports run out, we get
an EADDRNOTAVAIL error from bind() rather than an EADDRINUSE error
from connect(). Use small port range to exhaust ports and see which
error happens.

Reviewed by: tuexen, glebius, melifaro
Differential Revision: https://reviews.freebsd.org/D36056
MFC after: 3 days (with 637f317c6d9c)