selftests/tc-testing: Test htb_dequeue_tree with deactivation and row emptying

Ensure that any deactivation and row emptying that occurs
during htb_dequeue_tree does not cause a kernel panic.
This scenario originally triggered a kernel BUG_ON, and
we are checking for a graceful fail now.

Signed-off-by: William Liu <will@willsroot.io>
Signed-off-by: Savino Dicanosa <savy@syst3mfailure.io>
Link: https://patch.msgid.link/20250717022912.221426-1-will@willsroot.io


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