selftests: tc-testing: Add TBF with SKBPRIO queue length corner case test

Add a test case to validate the interaction between TBF and SKBPRIO queueing
disciplines, specifically targeting queue length accounting corner cases.

This test complements the fix for the queue length accounting issue in the
SKBPRIO qdisc. This is still best-effort, as timing and manipulating enqueue
and dequeue from user-space is very hard.

Cc: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/20250329222536.696204-3-xiyou.wangcong@gmail.com


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