selftests: netfilter: nft_nat.sh: move to lib.sh infra

Use busywait helper to wait until socat listener is up to avoid "sleep" calls.
This reduces script execution time slighty (12s to 7s).

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-16-fw@strlen.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>