selftests/tc-testing: add selftests for pfifo_fast qdisc

Test 900c: Create pfifo_fast with default setting
Test 7470: Dump pfifo_fast stats
Test b974: Replace pfifo_fast with different handle
Test 3240: Delete pfifo_fast with valid handle
Test 4385: Delete pfifo_fast with invalid handle

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>