selftests/tc-testing: Create test cases for adding qdiscs to invalid qdisc parents

As described in a previous commit [1], Lion's patch [2] revealed an ancient
bug in the qdisc API. Whenever a user tries to add a qdisc to an
invalid parent (not a class, root, or ingress qdisc), the qdisc API will
detect this after qdisc_create is called. Some qdiscs (like fq_codel, pie,
and sfq) call functions (on their init callback) which assume the parent is
valid, so qdisc_create itself may have caused a NULL pointer dereference in
such cases.

This commit creates 3 TDC tests that attempt to add fq_codel, pie and sfq
qdiscs to invalid parents

- Attempts to add an fq_codel qdisc to an hhf qdisc parent
- Attempts to add a pie qdisc to a drr qdisc parent
- Attempts to add an sfq qdisc to an inexistent hfsc classid (which would
  belong to a valid hfsc qdisc)

[1] https://lore.kernel.org/all/20250707210801.372995-1-victor@mojatatu.com/
[2] https://lore.kernel.org/netdev/d912cbd7-193b-4269-9857-525bee8bbb6a@gmail.com/



Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20250712145035.705156-1-victor@mojatatu.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>