ZDTM tests redirect standart descriptors to /dev/null

v2: * Skip tty only if it's a standard descriptor.
    * No strcmps on names.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>