p11: handle multiple certs during auth with OpenSSL
This patch adds missing code already available in the NSS version to
select a certificate for authentication if multiple certificates are
available on the Smartcard. A unit test to check this feature is added
as well.
Related to https://pagure.io/SSSD/sssd/issue/3489
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>