selftests: nft_nat: add udp hole punch test case

Add a test case that demonstrates port shadowing via UDP.

ns2 sends packet to ns1, from source port used by a udp service on the
router, ns0.  Then, ns1 sends packet to ns0:service, but that ends up getting
forwarded to ns2.

Also add three test cases that demonstrate mitigations:
1. disable use of $port as source from 'unstrusted' origin
2. make the service untracked.  This prevents masquerade entries
   from having any effects.
3. add forced PAT via 'random' mode to translate the "wrong" sport
   into an acceptable range.

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