zdtm/socket-tcp: execute iptables with the -w option to wait the xtables lock

================= Run zdtm/static/socket-tcp-nfconntrack in h ==================
Start test
Test is SUID
./socket-tcp-nfconntrack --pidfile=socket-tcp-nfconntrack.pid --outfile=socket-tcp-nfconntrack.out
Another app is currently holding the xtables lock. Perhaps you want to use the -w option?
Makefile:331: recipe for target 'socket-tcp-nfconntrack.pid' failed
make: *** [socket-tcp-nfconntrack.pid] Error 1

Reported-by: Mr Travis
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm/static/socket-tcp.c

@@ -69,7 +69,7 @@ int main(int argc, char **argv)
 	unshare(CLONE_NEWNET);
 	if (system("ip link set up dev lo"))
 		return 1;
-	if (system("iptables -A INPUT -i lo -p tcp -m state --state NEW,ESTABLISHED -j ACCEPT"))
+	if (system("iptables -w -A INPUT -i lo -p tcp -m state --state NEW,ESTABLISHED -j ACCEPT"))
 		return 1;
 	system("iptables -A INPUT -j DROP");
 #endif