selftests: mlxsw: RED: Test per-TC ECN counters

Add a variant of ECN test that uses qdisc marked counter (supported on
Spectrum-3 and above) instead of the aggregate ethtool ecn_marked counter.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>