selftests: router_vid_1: Add a diagram, fix coding style

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.

While at it, fix the list of tests so that the test names are enumerated
one at a line.

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>