This commit decouples StdlibUnittest from many details of
[Contiguous]Array, so tests can give useful feedback even in the
presence of a broken [Contiguous]Array implementation while we refactor.
Unfortunately, it wasn't practical to make _UnitTestArray use a
storage class other than _ContiguousArrayStorage[Base], so we still have
to watch out when making changes there.
Swift SVN r22875
ContiguousArray is a simpler component, thus less prone to breakage.
Builting the unit testing framework atop broken components is a very bad
idea, so let's not.
This is a reinstatement of an earlier commit, plus changes to make the
validation tests work.
Swift SVN r22867
