selftests: netfilter: switch zone stress to socat

centos9 has nmap-ncat which doesn't like the '-q' option, use socat.
While at it, mark test skipped if needed tools are missing.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>