selftests: netfilter: nft_nat.sh: add test for reverse clash with nat

This will fail without the previous bug fix because we erronously
believe that the clashing entry went way.

However, the clash exists in the opposite direction due to an
existing nat mapping:
 PASS: IP statless for ns2-LgTIuS
 ERROR: failed to test udp ns1-x4iyOW to ns2-LgTIuS with dnat rule step 2, result: ""

This is partially adapted from test instructions from the below
ubuntu tracker.

Link: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2109889


Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: Shaun Brady <brady.1345@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>