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.
I wasn't sure if implicit returns were supposed to work in future. I suppose not. Shall I update the docs without  'The transfer can be [...] implicit, when returning Unique from a function'?
https://github.com/ntrel/phobos/commit/96b18f589a27300c158a28a7d8ba58c26ccc7f90#diff-4e008aedb3026d4a84f58323e53bf017L53