protoc_plugin does not test dart_name extension handling

https://github.com/google/protobuf.dart/blob/8792f2aca21c087e9c061cb40aaa88af3cc6c3e4/protoc_plugin/lib/names.dart#L457-L458

The line above handles [`dart_name` extension][1] and it's currently not tested in protoc_plugin tests. I can change that line to

```dart
var name = _nameOption(field);
if (name.isNotEmpty) {
assert(false);
}
```

and I can still compile all of the protos used in the tests to Dart and all tests pass.

Not too important, but conversion of names to snake_case and then back during serialization seems quite delicate, and we have a few attributes/properties related to names of fields (`dart_name`, `json_name`). It would be good if we had good coverage here so that (1) code could be refactored without breakage (2) I could look at the tests for the intended behavior when the code gets tricky.

[1]: https://github.com/google/protobuf.dart/blob/8792f2aca21c087e9c061cb40aaa88af3cc6c3e4/protoc_plugin/protos/dart_options.proto#L76-L78