test/secmemtest: test secure memory only if it is implemented

Fixes #7322

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/7351)

(cherry picked from commit 8529b156421805415bc7b17758255394de580c61)