Phobos should provide unit testing faciilities

[Quirin Schroll]

In [*Taming Immutable Data Types in D with 
librebindable*](https://www.youtube.com/watch?v=eGX_fxlig8I) by 
Mathis Beer, in the Q&A, it was pointed out that it’s quite 
common to forget testing template code against types that do 
exotic (yet valid) things, e.g. `@disable this()`, 
define/`@disable` the copy constructor, contain qualified 
members, be immutable, etc.

One way to mitigate this would be a Phobos module (name it 
`std.testing` or something) that – possibly among other things – 
defines aptly named types that are irregular for the sole purpose 
of being used as examples, together with various AliasSeqs that 
group types that belong together somehow. Then, one could do:

```D
unittest
{
     import std.testing;
     static foreach (T; IrregularStructs)
     {
         T obj; // can default construct? T = NoDefaultCtor fails.
         auto obj2 = obj; // can copy? T = NoCopyCtor fails.
         // etc.?
     }
}
```

Anyone can find *some* irregular types and test against them, but 
looking into *every* corner requires extensive knowledge about 
the language’s niches.