Improve testing for ApiFormatBase subclasses

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.

Bug: T85236
Change-Id: I9cdf896e7070ed51e42625d61609ad9ef91cd567