selftests/tc-testing: Add test for HFSC queue emptying during peek operation

Add a selftest to exercise the condition where qdisc implementations
like netem or codel might empty the queue during a peek operation.
This tests the defensive code path in HFSC that checks the queue length
again after peeking to handle this case.

Based on the reproducer from Gerrard, improved by Jamal.

Reported-by: Gerrard Tai <gerrard.tai@starlabs.sg>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20250417184732.943057-4-xiyou.wangcong@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>