Both APIs conceptually do the same thing: "use these CA certs for
verification." However, set_ca_cert_store() falls through to the else
branch in load_certs() where system certs are added to the user's
custom store, defeating the purpose of certificate pinning.
This change makes set_ca_cert_store() behave consistently with
set_ca_cert_path() by checking ca_cert_store_ before loading system
certificates.
Added test to verify system certs are not loaded when custom store is set.
Co-authored-by: Your <you@example.com>
