selftests/mptcp: add diag interface tests

basic functional test, triggering the msk diag interface
code. Require appropriate iproute2 support, skip elsewhere.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>