selftests: netfilter: nft_queue.sh: sctp coverage

Test that nfqueue with and without GSO process SCTP packets correctly.

Joint work with Florian and Pablo.

Signed-off-by: Antonio Ojea <aojea@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>