Add unit-tests for DOCX preservation of 'table cell redline'

Added unit-tests to make sure that 'table cell redlines'
(e.g. - table cell was inserted \ removed) are being round
tripped correctly from a DOCX file.

Change-Id: I773cb51a39654d2640200489d5199361b56396c7
Reviewed-on: https://gerrit.libreoffice.org/7878
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>