selftests/tc-testing: add selftests for tbf qdisc

selftests/tc-testing: add selftests for tbf qdisc

Test 6430: Create TBF with default setting
Test 0518: Create TBF with mtu setting
Test 320a: Create TBF with peakrate setting
Test 239b: Create TBF with latency setting
Test c975: Create TBF with overhead setting
Test 948c: Create TBF with linklayer setting
Test 3549: Replace TBF with mtu
Test f948: Change TBF with latency time
Test 2348: Show TBF class

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