selftests/tc-testing: add selftests for skbprio qdisc

Test 283e: Create skbprio with default setting
Test c086: Create skbprio with limit setting
Test 6733: Change skbprio with limit setting
Test 2958: Show skbprio class

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