selftests/tc-testing: add tests for qdisc_tree_reduce_backlog

selftests/tc-testing: add tests for qdisc_tree_reduce_backlog

Add 3 tests to check for the expected behaviour of
qdisc_tree_reduce_backlog in special scenarios.

- The first test checks if the qdisc class is notified of deletion for
major handle 'ffff:'.
- The second test checks the same as the first test but with 'ffff:' as the root
qdisc.
- The third test checks if everything works if ingress is active.

Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Acked-by: Cong Wang <cong.wang@bytedance.com>
Link: https://patch.msgid.link/20241101143148.1218890-1-pctammela@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>