selftests: mlxsw: Add a test for leftover DSCP rule

Commit dedfde2f ("mlxsw: spectrum_dcb: Configure DSCP map as the last
rule is removed") fixed a problem in mlxsw where last DSCP rule to be
removed remained in effect when DSCP rewrite was applied.

Add a selftest that covers this problem.

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