zdtm.sh: check snapshot functionality

Only last iteration stops processes, all other iterations make
snapshots. Processes are restored from the last snapshot.

bash -x test/zdtm.sh -s -i 3

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>