zdtm: Static pstree test

I'm waiting for Andrey's

* pthread join fix
* infrastructure for fokring in zdtm

to extend it with threads.

Besides, we/me need to fix the seizing of dynamically forking tasks
to write a transition pstree test.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>