tests: fix up meta l4proto change for ip6 family

tests: fix up meta l4proto change for ip6 family

After previous commit nft generates meta l4proto for ipv6 dependencies
instead of checking the (first) nexthdr value.

This fixes up all tests cases accordingly except one which fails with

ip6/reject.t: ... 12: 'ip6 nexthdr 6 reject with tcp reset' mismatches 'meta l4proto 6 reject with tcp reset'
This will be fixed by removing the implicit dependency in a followup patch.

Signed-off-by: Florian Westphal <fw@strlen.de>