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

use checktool helper where applicable.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240418152744.15105-4-fw@strlen.de


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