selftests/tc-testing: Add a test case to cover basic HTB+FQ_CODEL case

Integrate the reproducer from Alan into TC selftests and use scapy to
generate TCP traffic instead of relying on ping command.

Cc: Alan J. Wylie <alan@wylie.me.uk>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Link: https://patch.msgid.link/20250428232955.1740419-3-xiyou.wangcong@gmail.com


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