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>

test/zdtm.sh

@@ -420,7 +420,7 @@ start_test()
 	[ -z "$PIDNS" ] || cd -
 
 	PID=`cat "$TPID"` || return 1
-	if ! ps -p $PID ; then
+	if ! kill -0 $PID ; then
 		echo "Test failed to start"
 		return 1
 	fi