certmap testing: fix wrong cert construction
    
    `bytes` instances have no `.format()`, we can simply base64 decode
    the certificate and load it as DER instead.
    
    https://pagure.io/freeipa/issue/7131
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>