selftests/tc-testing: Add a test case for FQ_CODEL with HTB parent

selftests/tc-testing: Add a test case for FQ_CODEL with HTB parent

Add a test case for FQ_CODEL with HTB parent to verify packet drop
behavior when the queue becomes empty. This helps ensure proper
notification mechanisms between qdiscs.

Note this is best-effort, it is hard to play with those parameters
perfectly to always trigger ->qlen_notify().

Cc: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20250403211636.166257-2-xiyou.wangcong@gmail.com
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>