Fix comments, add new test.

Fix comments in ssltest.c: return value of 0 now means extension is
omitted and add_cb is not called for servers if the corresponding
extension is absent in ClientHello.

Test add_cb is not called if extension is not received.
Reviewed-by: Emilia Käsper <emilia@openssl.org>
(cherry picked from commit f47e2039)