selftests/tc-testing: add qdisc hierarchy depth tests

selftests/tc-testing: add qdisc hierarchy depth tests

Add tdc coverage for the qdisc hierarchy depth limit.

The tests verify that the deepest allowed hierarchy can still be created
and that adding another child qdisc below it is rejected.

Signed-off-by: Zijie Huang <milkory@outlook.com>
Assisted-by: Codex:gpt-5.4
Signed-off-by: Ren Wei <enjou1224z@gmail.com>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/5cc2d26a7c8e553759cdd29a3116f843fabc25ba.1785434373.git.milkory@outlook.com
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>