Add tests to chord::routing::identifier module

Tests are missing for the following implementations:

- [ ] `Identifier::as_bytes`
- [ ] `SocketAddrV4::identifier`
- [ ] `SocketAddrV6::identifier`
- [ ] `SocketAddr::identifier`
- [ ] `Key::identifier`
- [ ] `IdentifierValue::identifier` and `IdentifierValue::deref`