[WIP/RFC] Add unit tests for buffer.c and fileio.c

> Well, we are then turning around and telling gen_expand_wildcards that it isn't :).

Hehe, I see what you mean. But `gen_expand_wildcards` expects an array of strings, and, well, in fact @war1025's solution is more or less the right one. I think I would've gone for something like:

```c
char_u *ptr = NameBuff;  // perhaps a cast is necessary here
gen_expand_wildcards(1, &ptr, &fcount, ...);
```

It looks a bit less alien (though I'm not sure if it works). But the downside is that it's less obvious that something tricky is going on.

```