selftests: qdiscs: Add test coverage for ETS Qdisc

Add TDC coverage for the new ETS Qdisc.

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