This test lacks a topology diagram, making the setup not obvious.
Add one.

Cc: David Ahern <dsahern@gmail.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>