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
Merge "Improve testing for ApiFormatBase subclasses"