Includes unit testing infrastructure for .txt export too + the actual unit
test.

Change-Id: I19a32955bbc9b97449b4240917fe2505bc3dd54c
Reviewed-on: https://gerrit.libreoffice.org/37295Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan Holesovsky <kendy@collabora.com>