selftests/tc-testing: add selftests for atm qdisc

Test 7628: Create ATM with default setting
Test 390a: Delete ATM with valid handle
Test 32a0: Show ATM class
Test 6310: Dump ATM stats

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