selftests/tc-testing: add selftests for htb qdisc

Test 0904: Create HTB with default setting
Test 3906: Create HTB with default-N setting
Test 8492: Create HTB with r2q setting
Test 9502: Create HTB with direct_qlen setting
Test b924: Create HTB with class rate and burst setting
Test 4359: Create HTB with class mpu setting
Test 9048: Create HTB with class prio setting
Test 4994: Create HTB with class ceil setting
Test 9523: Create HTB with class cburst setting
Test 5353: Create HTB with class mtu setting
Test 346a: Create HTB with class quantum setting
Test 303a: Delete HTB with handle

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>