Fix Issue 13068 - std.typecons.Unique should disable postblit

https://issues.dlang.org/show_bug.cgi?id=13068

* Fix struct unittest.
* Fix class unittest.
* Disable `Unique` postblit.
Oh, I guess see why you left it in now. My comment (heh) was only addressed at leaving a piece of commented code out in there without further explanation, which I often find confusing when perusing code I didn't write myself later.