selftests/tc-testing: add selftests for dsmark qdisc

selftests/tc-testing: add selftests for dsmark qdisc

Test 6345: Create DSMARK with default setting
Test 3462: Create DSMARK with default_index setting
Test ca95: Create DSMARK with set_tc_index flag
Test a950: Create DSMARK with multiple setting
Test 4092: Delete DSMARK with handle
Test 5930: Show DSMARK 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>