iptables-test: fix netns test

iptables-test: fix netns test

The libxt_rateest test always fails because dependent command is not
executed in netns.

(@iptables -I INPUT -j RATEEST --rateest-name RE1 --rateest-interval \
 250.0ms --rateest-ewmalog 500.0ms)

After this path, adding netns command is executed first.
Then test commands are executed.

Fixes: 0123183f43a9 ("iptables-test: add -N option to exercise netns removal path")
Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>