remove all atexit() tests in shlibloadtest

remove all atexit() tests in shlibloadtest

The shlibloadtest used atexit() handler to verify
library pinning works as expected. The libcrypto
no longer arms atexit handler which also used to
fire upon shlib unload. We can not use the atexit
mechansim to test shared library pinning.

If the shlibload test does not crash on exit, then
library pinning must work.

Fixes openssl/project#1869

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
MergeDate: Fri Feb 13 15:15:04 2026
(Merged from https://github.com/openssl/openssl/pull/29987)