Enable test for issue 8231

http://d.puremagic.com/issues/show_bug.cgi?id=8321

While ago, I posted both compiler and phobos fix to fix issue 8231.
https://github.com/D-Programming-Language/dmd/issues/1032
https://github.com/D-Programming-Language/phobos/pull/656

And now, whole blocker issues are completely fixed (as I said [here](https://github.com/D-Programming-Language/dmd/pull/1032#issuecomment-14370573)), so I'd enable the unittest.