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

Only relevant change is that netns names have random suffix names,
i.e. its safe to run this in parallel with other tests.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-5-fw@strlen.de


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