std.experimental.testing PR review

[Atila Neves via Digitalmars-d]

Original library: http://code.dlang.org/packages/unit-threaded
PR: https://github.com/D-Programming-Language/phobos/pull/3207

It's my first Phobos PR, I tried reading the wiki and doing 
what's required but bear with me if I've screwed up somehow.

I wasn't sure whether or not to split the PR. In the end I just 
took the existing library, edited it a lot and got it ready for 
review.

Unit test blocks can be named with @Name. They execute in 
parallel by default but that's only if the default runner is used.

Atila