selftests: tc-testing: add test for qfq with stab overhead

A packet with stab overhead greater than QFQ_MAX_LMAX should be dropped
by the QFQ qdisc as it can't handle such lengths.

Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Tested-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>