Add a shutdown sequence regress test.

Add a shutdown sequence regress test.

Some software relies on SSL_shutdown() returning 0 (indicating close-notify
sent) before returning 1 on a subsequent call (indicating close-notify sent
and received). It is worth noting that there is no guarantee that this will
occur in normal operation, as the peer could send a close-notify prior to
SSL_shutdown() being called.

This is currently failing for TLSv1.3.