Refactor Callback Tests for Improved Memory Management

Refactor Callback Tests for Improved Memory Management

Refactor the callback test code to replace global variables with local structures, enhancing memory management and reducing reliance on redundant cleanup logic.

Using a local struct containing a magic number and result flag to ensure the correct handling of user data and to verify that the callback function is invoked at least once during the test.

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25330)