selftests/mptcp: add diag listen tests

Check dumping of mptcp listener sockets:
1. filter by dport should not return any results
2. filter by sport should return listen sk
3. filter by saddr+sport should return listen sk
4. no filter should return listen sk

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