sss_nss_getlistbycert: return results from multiple domains
    
    Currently only the results from one domain were returned although all
    domains were searched and the results were available. Unit tests are
    updated to cover this case as well.
    
    Resolves https://pagure.io/SSSD/sssd/issue/3393
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>