selftests/tls: add a litmus test for the socket reuse through shutdown

Make sure that shutdown never works, and at the same time document how
I tested to came to the conclusion that currently reuse is not possible.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>