selftests: mlxsw: RED: Add selftests for the mark qevent

Add do_mark_test(), which is to do_ecn_test() like do_drop_test() is to
do_red_test(): meant to test that actions on the RED mark qevent block are
offloaded, and executed on ECN-marked packets.

The test splits install_qdisc() into its constituents, install_root_qdisc()
and install_qdisc_tcX(). This is in order to test that when mirroring is
enabled on one TC, the other TC does not mirror.

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