TEST_NEGCACHE: Test that "root" is always added to ncache
    
    Simply modify test_sss_ncache_prepopulate() in order to ensure that
    "root" user and group are always added to the negative cache, no matter
    whether they're set as part of the filter_users or filter_groups
    options.
    
    Related: https://pagure.io/SSSD/sssd/issue/3460
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit e54764d62bfcc48770d9b2578132979aa58636e5)
    (cherry picked from commit cfc62a3cc9683a78e5a5ccb31c7e5094c1b91a91)