selftests/tc-testing: add selftests for gred qdisc

Test 8942: Create GRED with default setting
Test 5783: Create GRED with grio setting
Test 8a09: Create GRED with limit setting
Test 48cb: Create GRED with ecn setting
Test 763a: Change GRED setting
Test 8309: Show GRED class

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