selftests: netfilter: nft_conntrack_helper.sh: test to lib.sh infra

prefer socat over nc, nc has too many incompatible versions around.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-13-fw@strlen.de


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