selftests: mlxsw: Add a test for un/offloadable qdisc trees

This checks that various qdisc configurations either are or are not
offloaded.

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>