swift234

[stdlib] Add _UnitTestArray[Buffer] temporarily

We're going to replace StdlibUnittest's use of ContiguousArray with
_UnitTestArray so that we can work on (and potentially break)
_ContiguousArray and still get useful test results.  When refactoring is
complete, we can optionally replace StdlibUnittest's use of
_UnitTestArray with ContiguousArray, or move the decoupled component
into the StdlibUnittest module.

Swift SVN r22874