selftests/tc-testings: add tunnel_key action deleting test case

Test 3671: Delete tunnel_key set action with valid index
Test 8597: Delete tunnel_key set action with invalid index

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