selftests: netfilter: extend flowtable test script with dnat rule

NAT test currently covers snat (masquerade) only.

Also add a dnat rule and then check that a connecting to the
to-be-dnated address will work.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>