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

Also lower ping interval, wait times (helpers get called several times)
and set nodad for ipv6 addresses: 20s down to 4s.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-14-fw@strlen.de


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