selftests: netfilter: nft_flowtable.sh: move test to lib.sh infra

Use socat, the different nc implementations have too much variance wrt.
supported options.

Avoid sleeping until listener is up, use busywait helper for this,
this also greatly reduces test duration.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-15-fw@strlen.de


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