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

Use busywait helper instead of unconditional sleep, reduces run time
from 6m to 2:30 on my system.

The busywait helper calls the function passed to it as argument; disable
the shellcheck test for unreachable code, it generates many (false)
warnings here.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240423130604.7013-2-fw@strlen.de


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