selftests/tc-testing: add selftests for cbs qdisc

Test 1820: Create CBS with default setting
Test 1532: Create CBS with hicredit setting
Test 2078: Create CBS with locredit setting
Test 9271: Create CBS with sendslope setting
Test 0482: Create CBS with idleslope setting
Test e8f3: Create CBS with multiple setting
Test 23c9: Replace CBS with sendslope setting
Test a07a: Change CBS with idleslope setting
Test 43b3: Delete CBS with handle
Test 9472: Show CBS class

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>