TEST: Refactor of test_responder_cache_req.c
    
    This patch only defines constant TEST_USER_PREFIX. So code will be more
    redeable.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2730
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit b0e8c1802557645e2ff6a88c54c520b0f0ff9ebb)