tools/testing: add a selftest for SO_NETNS_COOKIE

Make sure that SO_NETNS_COOKIE returns a non-zero value, and
that sockets from different namespaces have a distinct cookie
value.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>