selftests/tc-testings: add selftests for xt action

Test 2029: Add xt action with log-prefix
Test 3562: Replace xt action log-prefix
Test 8291: Delete xt action with valid index
Test 5169: Delete xt action with invalid index
Test 7284: List xt actions
Test 5010: Flush xt actions
Test 8437: Add xt action with duplicate index
Test 2837: Add xt action with invalid index

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