tests: t2200-dos-label-recog.sh: adapt to work with larger sector size

tests: t2200-dos-label-recog.sh: adapt to work with larger sector size

Using a 100K image is inadequate for larger sector sizes, since it
triggers a bug whereby the device length (in sectors) smaller than
the cylinder_size: from dos.c:
  if (!ped_geometry_init (&start_geom, dev, cylinder_size,
      dev->length - cylinder_size))
That would pass a negative value for the 3rd argument.
* tests/t2200-dos-label-recog.sh: Use an image of size 10MB, not 100K.