selftests/tc-testing: add selftests for plug qdisc

selftests/tc-testing: add selftests for plug qdisc

Test 3289: Create PLUG with default setting
Test 0917: Create PLUG with block setting
Test 483b: Create PLUG with release setting
Test 4995: Create PLUG with release_indefinite setting
Test 389c: Create PLUG with limit setting
Test 384a: Delete PLUG with valid handle
Test 439a: Replace PLUG with limit setting
Test 9831: Change PLUG with limit setting

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