swift234

[StdlibUnittest] Use _UnitTestArray now

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