selftests: mlxsw: Add router test

This test aims for both stand alone and internal usage by the resource
infra. The test receives the number routes to offload and checks:
- The routes were offloaded correctly
- Traffic for each route.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>