selftests: mlxsw: Add scale test for resources

Add a scale test capable of validating that offloaded network
functionality is indeed functional at scale when configured to
the different KVD profiles available.

Start by testing offloaded routes are functional at scale by
passing traffic on each one of them in turn.

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