src/test.cc: wrong field printed

src/test.cc, around line 95:

```
printf ("cluster %d glyph 0x%x at   (%d,%d)+(%d,%d)\n",
    info->cluster,
    info->codepoint,
    pos->x_offset,
    pos->x_offset,
    pos->x_advance,
    pos->y_advance);
```

Note that pos->x_offset is printed twice, I suspect that the second was intended to be pos->y_offset.
