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

@jszakmeister Just to be sure, modified test code:

```c
#include <stdio.h>                        
static int foo[1024*1024*100];            
int main(int argc, char *argv[]) {        
  getchar();                              
  for(int i = 0; i < 1024*1024*100; i++) {
    foo[i] = i;                           
  }                                       
  getchar();                              
  return 0;                               
}                 
```

Before pressing a key, RES memory is ~300 bytes. After pressing key, it jumps up to 400MB.