test: add decryption test for isakmp with certificates

This was suggested in review 2297. Capture and key are from bug 7951.

Bug: 7951
Change-Id: I820c5e839b20ec464cc1be438633d5311f657fb2
Signed-off-by: Alex Badea <abadea@ixiacom.com>
Reviewed-on: https://code.wireshark.org/review/4143
Reviewed-by: Evan Huus <eapache@gmail.com>