TEST: New tests for negative caching of locals
    
    Negative cache has special timeout for local entities (users, groups).
    This patch adds new tests for such behaviour. The tests are based on
    cwrap.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2928
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>