selftests: mlxsw: qos_dscp_router: Test no DSCP rewrite after pedit

When DSCP is updated through an offloaded pedit action, DSCP rewrite on
egress should be disabled. Add a test that check that it is so.

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