zdtm: exit a test if the "root" process died

Each test has a process which controls other processes and
if it dies the other processes have to be killed.

travis-ci: success for zdtm: exit a test if the "root" process died
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>