selftests: netfilter: add simple masq/redirect test cases

[ Upstream commit 98bfc3414bda335dbd7fec58bde6266f991801d7 ]

Check basic nat/redirect/masquerade for ipv4 and ipv6.

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