TESTS: fixed regression in cmocka/test_negcache_2.c
Fixed regression in test introduced in 2b564f8 (PR 786)
Test was relying on hardcoded values of non local users and groups.
Test was changed to find those in runtime.
Resolves: https://pagure.io/SSSD/sssd/issue/3964
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>