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

Intentional changes:
- Use socat instead of netcat
- Use a temporary file instead of pipe, else packets do not match
  "-m string" rules, multiple writes to the pipe cause multiple packets,
  but this needs only one to work.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240418152744.15105-6-fw@strlen.de


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