selftests: netfilter: nft_tproxy.sh: adjust to socat changes

selftests: netfilter: nft_tproxy.sh: adjust to socat changes

Like e65d8b6f3092 ("selftests: drv-net: adjust to socat changes") we
need to add shut-none for this test too.

The extra 0-packet can trigger a second (unexpected) reply from the server.

Fixes: 7e37e0eacd22 ("selftests: netfilter: nft_tproxy.sh: add tcp tests")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20260408152432.24b8ad0d@kernel.org/
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://patch.msgid.link/20260409224506.27072-1-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>