zdtm.sh: add a test directory name in a path to images

Otherwise static/socket-tcp and streaming/socket-tcp have the same path
to images and they can affect each other.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
pstree: Introduce task_alive() helper

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
pstree: Use task_alive() instead of switch()-es

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>