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

Also do shellcheck cleanups here, no functional changes intended.
When running tests via vng tool, the packetpath insertion test fails:
dd: failed to open '/dev/stdout': Device or resource busy

Just omit 'of=' and this will work as intended.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240418152744.15105-5-fw@strlen.de


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