zdtm.sh: wait page-server by a proper way

page-server is forked and the child is running in background, so a bash
variable $! contains PID of the last command, which is executed in
background.

Currently it's tcpdump and zdtm.sh is waiting it and never returns back.

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