selftests: mlxsw: Add test for ip_forward_update_priority

Verify that with that sysctl turned off, DSCP prioritization and rewrite
works the same way as in qos_dscp_bridge test. However when the sysctl
is charged, there should be a reprioritization after routing stage,
which will be observed by a different DSCP rewrite on egress.

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