selftests: netfilter: conntrack_resize.sh: extend resize test

Extend the resize test:
 - continuously dump table both via /proc and ctnetlink interfaces while
   table is resized in a loop.
 - if socat is available, send udp packets in additon to ping requests.
 - increase/decrease the icmp and udp timeouts while resizes are happening.
   This makes sure we also exercise the 'ct has expired' check that happens
   on conntrack lookup.

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