Move the ability to load the dasync engine into ssltestlib.c

Move the ability to load the dasync engine into ssltestlib.c

The sslapitest has a helper function to load the dasync engine which is
useful for testing pipelining. We would like to have the same facility
from sslbuffertest, so we move the function to the common location
ssltestlib.c

Follow on from CVE-2024-4741

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24395)

(cherry picked from commit 0544c21a22f4d787e6f31d35e8f980402ac90a6d)