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

Is there actually a good reason why `NameBuff` isn't statically allocated like `IOBuff` and `msg_buf`. Their definitions:

```c
EXTERN char_u   IObuff[IOSIZE];         /* sprintf's are done in this buffer,
                                           size is IOSIZE */
EXTERN char_u   *NameBuff;              /* file names are expanded in this
                                         * buffer, size is MAXPATHL */
EXTERN char_u msg_buf[MSG_BUF_LEN];     /* small buffer for messages */
```

@justinmk @tarruda @philix @ZyX-I Any of you guys know what's going on in (neo)vim's belly?