selftests/tc-testing: verify that a qdisc can be grafted onto a taprio class

selftests/tc-testing: verify that a qdisc can be grafted onto a taprio class

The reason behind commit af7b29b1deaa ("Revert "net/sched: taprio: make
qdisc_leaf() see the per-netdev-queue pfifo child qdiscs"") was that the
patch it reverted caused a crash when attaching a CBS shaper to one of
the taprio classes. Prevent that from happening again by adding a test
case for it, which now passes correctly in both offload and software
modes.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Pedro Tammela <pctammela@mojatatu.com>
Link: https://lore.kernel.org/r/20230807193324.4128292-12-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>