selftests: netfilter: add conntrack clash resolution test case

Add a dedicated test to exercise conntrack clash resolution path.
Test program emits 128 identical udp packets in parallel, then reads
back replies from socat echo server.

Also check (via conntrack -S) that the clash path was hit at least once.
Due to the racy nature of the test its possible that despite the
threaded program all packets were processed in-order or on same cpu,
emit a SKIP warning in this case.

Two tests are added:
 - one to test the simpler, non-nat case
 - one to exercise clash resolution where packets
   might have different nat transformations attached to them.

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