safe pure unittest

[simendsjo via Digitalmars-d-learn]

This is the first time I've seen attributes on unittests:
https://github.com/D-Programming-Language/phobos/pull/2349/files#diff-ba05e420ac1da65db044e79304d641b6R179

Has this always been supported? I guess it's good practice to add these
on unittests too, but does people even know about this feature? And are
there any cons to doing this?