socket tests: Add a test which calls listen() twice on an lb socket

This exercises commit 06bf119f265c ("sockets/tcp: quick fix for
regression with SO_REUSEPORT_LB")

Reviewed by: glebius
MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D48702