std.unittests for (final?) review

[Walter Bright]

Jens Mueller wrote:
> Unit testing can be implemented on top of the
> language and shouldn't be put into it. Somehow I have the feeling that
> too often one tries to extend the language even though the feature could
> be implemented in a library.

On the other hand, the built-in D unit test ability has been a huge success.

A unit test facility that is not used is worthless, no matter how capable it is. 
The advantage of it being simple and built-in is it gets used, and I think 
there's strong evidence that this is true for D.