tests: add test for cert_to_ssh_key request
    
    This patch adds cmocka base until tests for the new cert_to_ssh_key
    request
    
    There seems to be a memory leak in bash if a called binary returns an
    error code. Since p11_child is called via a libtool wrapper in the unit
    tests and returns an error code if a certificate is invalid this will be
    reported during the CI valgrind run. To make the CI pass here a
    suppression is added.
    
    Related to https://pagure.io/SSSD/sssd/issue/3489
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>