Use reported short conn id len in qtestlib

Use reported short conn id len in qtestlib

Use the new short conn id internal api to record and use the connections
short conn id len when decoding packets in qtestlib

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26592)