tests/050: fix a supurious failure

For a fast CPU which can write 100M to memory far less than 1 second, we cannot
sleep before iptable tricks and the logger might not flush the log yet before we
grep it so we need safely sleep for 1 second.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>