selftests: mlxsw: Test offloadability of root TBF

TBF can be used as a root qdisc, with the usual ETS/RED/TBF hierarchy below
it. This use should now be offloaded. Add a test that verifies that it is.

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