TEST: Refactor of test_responder_cache_req.c
    
    We need little more in background of responder_cache_req tests. There
    will be tests which will use three test users. This patch add support
    for it.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2730
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>