selftests: netfilter: add a vrf+conntrack testcase

Rework the reproducer for the vrf+conntrack regression reported
by Eugene into a selftest and also add a test for ip masquerading
that Lahav fixed recently.

With net or net-next tree, the first test fails and the latter
two pass.

With 09e856d5 ("vrf: Reset skb conntrack connection on VRF rcv")
reverted first test passes but the last two fail.

A proper fix needs more work, for time being a revert seems to be
the best choice, snat/masquerade did not work before the fix.

Link: https://lore.kernel.org/netdev/378ca299-4474-7e9a-3d36-2350c8c98995@gmail.com/T/#m95358a31810df7392f541f99d187227bc75c9963


Reported-by: Eugene Crosser <crosser@average.org>
Cc: Lahav Schlesinger <lschlesinger@drivenets.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>