selftests: router.sh: Add a diagram

It is customary for selftests to have a comment with a topology diagram,
which serves to illustrate the situation in which the test is done. This
selftest lacks it. Add it.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>