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

The setup_ns helper makes the netns names random, so replace nsX with $nsX
everywhere.

Replace nc with socat, otherwise script fails on my system due to
incompatible nc versions ("nc: cannot use -p and -l").

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-11-fw@strlen.de


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