zdtm: Replace `ps -p` with `kill -0`

The -p argument to `ps` is not supported by Busybox v1.19.4. Sending
signal 0 with the `kill` command is an alternative way to check
whether a process is still running.

http://www.linuxjournal.com/content/monitoring-processes-kill-0



Signed-off-by: Christopher Covington <cov@codeaurora.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>