selftests/tc-testing: Add qdisc limit trimming tests

Added new test cases for FQ, FQ_CODEL, FQ_PIE, and HHF qdiscs to verify queue
trimming behavior when the qdisc limit is dynamically reduced.

Each test injects packets, reduces the qdisc limit, and checks that the new
limit is enforced. This is still best effort since timing qdisc backlog
is not easy.

Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>