Improve testing for ApiFormatBase subclasses
I7b37295e is going to be changing around how ApiResult works,
which is going to need corresponding changes in the formatters. So it
would probably be a good idea to have a decent starting point to catch
any breakage. The non-backwards-compatible changes to ApiFormatTestBase
shouldn't be a concern, as no extensions in Gerrit reference this class
or any /ApiFormat.*Test/ class.
This also fixes two small bugs in ApiFormatWddx (null handling and
spacing for non-fm slow path) discovered during testing, and works
around some HHVM wddx extension bugs.