Use zero ports in tests

[jmcph4]

Similarly to sigp/lighthouse#4704, some of our tests race on port number allocation. The main offenders are:

• discv5/src/service/test.rs

  Line 102 in 299638d

  async fn test_updating_connection_on_ping() {

• discv5/src/service/test.rs

  Line 169 in 299638d

  async fn test_connection_direction_on_inject_session_established() {

These both hardcode UDP ports (like 10001 and 10002). Locally, all it takes me is 3-4 runs of the test suite to induce failures due to AddrInUse. The specific failure case is always a panic when the port can't be allocated:

thread 'service::test::test_connection_direction_on_inject_session_established' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 98, kind: AddrInUse, message: "Address already in use" }', src/service/test.rs:58:14