Fix DocTests running in non-standard width terminal

The changes introduced with
  56976a127c081a6a008c81966360003a8711a319 make --columns default to
  terminal width, as returned by ioctl()
break the DocTests when run in a terminal with a width different
from the standard of 80 columns.
Merge pull request #337 from afh/pull/DocTest

Fix DocTests running in non-standard width terminal