selftests: mlxsw: Add test cases for local table route replacement

Test that routes in the main table do not replace identical routes in
the local table and that routes in the local table do replace identical
routes in the main table.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>