stun: stun test service use stun.cloudflare.com:3478

[cppcoffee]

Fix the issue where cargo test revealed that the response from stun1.l.google.com:19302 failed:

test client::client_test::test_client_with_stun_send_binding_request ... FAILED
test client::client_test::test_client_with_stun_send_binding_request_to_parallel ... FAILED

failures:

---- client::client_test::test_client_with_stun_send_binding_request stdout ----
Error: Stun(ErrAttributeNotFound)

---- client::client_test::test_client_with_stun_send_binding_request_to_parallel stdout ----
Error: Stun(ErrAttributeNotFound)


failures:
    client::client_test::test_client_with_stun_send_binding_request
    client::client_test::test_client_with_stun_send_binding_request_to_parallel

test result: FAILED. 62 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out; finished in 15.64s

error: test failed, to rerun pass `-p turn --lib`

instead, use Cloudflare's STUN service:

https://developers.cloudflare.com/realtime/turn/#service-address-and-ports