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>

selftests: forwarding: Add router test with VID 1

Previous patches made it possible to setup VLAN devices with VID 1 over
mlxsw ports. Verify this functionality actually works by conducting a
simple router test over VID 1.

Adding this test as a generic test since it can be run using veth pairs
and it can also be useful for other physical devices where VID 1 was
considered reserved (knowingly or not).

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