Fix a failure in sslapitest

Fix a failure in sslapitest

The SNI test in test_cert_cb_int() was always failing because it used
SSL_CTX_new() instead of SSL_CTX_new_ex() and was therefore not using the
correct libctx. PR #17739 amended the test to check the return value from
SSL_CTX_new() which made the failure obvious.

Fixes #17757

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/17758)