selftests/tc-testing: Create tests to exercise ets classes active list misplacements

selftests/tc-testing: Create tests to exercise ets classes active list misplacements

Add a test case for a bug fixed by Jamal [1] and for scenario where an
ets drr class is inserted into the active list twice.

- Try to delete ets drr class' qdisc while still keeping it in the
  active list
- Try to add ets class to the active list twice

[1] https://lore.kernel.org/netdev/20251128151919.576920-1-jhs@mojatatu.com/

Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20251208190125.1868423-2-victor@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>