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

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

Change-Id: I21e67b3ed8c98e8aba2c633dc6a549786ae44322
Reviewed-on: https://gerrit.libreoffice.org/7241
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>