Add unit-test for 'track changes - deleted paragraph mark' preservation

This is a unit-test added to complement the patch that added support
for the preservation of 'Track Changes - Deleted Paragraph Mark' from
a DOCX file.

Conflicts:
	sw/qa/extras/ooxmlexport/ooxmlexport.cxx
Reviewed on:
	https://gerrit.libreoffice.org/7240

Change-Id: I161bcd90ec891fad60dfd5b7c77edddfd90b7fa8