selftests: mptcp: add test cases for mptcp join tests with syn cookies

Also add test cases with MP_JOIN when tcp_syncookies sysctl is 2 (i.e.,
syncookies are always-on).

While at it, also print the test number and add the test number
to the pcap files that can be generated optionally.

This makes it easier to match the pcap to the test case.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>