tests: icmpX: fix expected output

tests: icmpX: fix expected output

both of these rules succeed, but they should fail instead.
nft removes the ip6 nexthdr' clause, but this is not correct, it is an
explicit test for the ipv6 nexthdr value.

Implicit dependencies use meta l4proto to skip extension headers
(if any), ipv6 nexthdr does not.

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