selftests/tc-testing: match any qdisc type

We should not always presume all kernels use pfifo_fast as the default qdisc.

For example, a fq_codel qdisk could have below output:
qdisc fq_codel 0: parent 1:4 limit 10240p flows 1024 quantum 1514 target 5ms interval 100ms memory_limit 32Mb ecn drop_batch 64

Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Peilin Ye <peilin.ye@bytedance.com>
Signed-off-by: Li Zhijian <zhijianx.li@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>