selftests/tc-testing: add selftests for mqprio qdisc

Test 9903: Add mqprio Qdisc to multi-queue device (8 queues)
Test 453a: Delete nonexistent mqprio Qdisc
Test 5292: Delete mqprio Qdisc twice
Test 45a9: Add mqprio Qdisc to single-queue device
Test 2ba9: Show mqprio 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>