selftests: netfilter: nft_nat_zones.sh: shellcheck cleanups

While at it: No need for iperf here, use socat.
This also reduces the script runtime.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240418152744.15105-8-fw@strlen.de


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