selftests/tc-testing: Add QFQ/CBS qlen underflow test

selftests/tc-testing: Add QFQ/CBS qlen underflow test

Since CBS was not calling reset for its child qdisc, there are scenarios
where it could cause an underflow on its parent's qlen/backlog. When the
parent is QFQ, a null-ptr deref could occur.

Add a test case that reproduces the underflow followed by a null-ptr
deref scenario.

Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>