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

https://issues.dlang.org/show_bug.cgi?id=13068

* Enable struct unittest (worked already)
* Enable class unittest with workaround of calling release when returning `Unique!class` (otherwise I get an access violation)
* Disable `Unique` postblit with workaround of calling release when returning `Unique!struct`

Note: this may break code that implicitly returned `Unique!struct`, but disabling the postblit is pretty necessary for usability (see bug report).