selftests/tc-testing: Add a test case for qdisc_tree_reduce_backlog()

selftests/tc-testing: Add a test case for qdisc_tree_reduce_backlog()

Integrate the test case provided by Mingi Cho into TDC.

All test results:

1..4
ok 1 ca5e - Check class delete notification for ffff:
ok 2 e4b7 - Check class delete notification for root ffff:
ok 3 33a9 - Check ingress is not searchable on backlog update
ok 4 a4b9 - Test class qlen notification

Cc: Mingi Cho <mincho@theori.io>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Link: https://patch.msgid.link/20250204005841.223511-5-xiyou.wangcong@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>