responder/negcache: avoid calling nsswitch NSS API
Changed "negcache_files.c::is_*_local_by_*()" to use functions from
"libnss_files" directly to check users (instead of calling glibc
NSS API).
Changed affected tests to avoid using NSS-wrapper and to use real
local user&group (otherwise tests were broken).
Resolves: https://pagure.io/SSSD/sssd/issue/3964
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
(cherry picked from commit 2b564f849a20289a857cf19bbfaa5c6eb8670bad)
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>