selftests: mlxsw: Test port shaper

TBF can be used as a root qdisc, in which case it is supposed to configure
port shaper. Add a test that verifies that this is so by installing a root
TBF with a ETS or PRIO below it, and then expecting individual bands to all
be shaped according to the root TBF configuration.

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