If a message is proto3, then the zero values still count as being set one the
field is in a oneof.

Add tests to confirm oneofs work as expected in both syntaxes.

* Add conformance test for zero fields in oneofs.
* Add failures to the "expected" files.

- Update all the new runner cases to handle a skip result as a skip and not a failure.
- Make ObjC use the new serialize_error result if it fails to generate the data.

[skip ci]

- Mark all conformance tests as now passing.