selftests: netfilter: extend nfqueue test case

add a test with re-queueing: usespace doesn't pass accept verdict,
but tells to re-queue to another nf_queue instance.

Also, make the second nf-queue program use non-gso mode, kernel will
have to perform software segmentation.

Lastly, do not queue every packet, just one per second, and add delay
when re-injecting the packet to the kernel.

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