Add tests for new extension code

Add tests for new extension code

Extend test_tls13messages to additionally check the expected extensions
under different options given to s_client/s_server.

Perl changes reviewed by Richard Levitte. Non-perl changes reviewed by Rich
Salz

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>