fdo#59886 unit tests for DOCX fixed date field import/export.

Checks fixed date fields are exported to docx and imported from
docx with correct initial value.

Change-Id: Ia6b6180b59806e624b03dc81a903ab1ac6d33307
Reviewed-on: https://gerrit.libreoffice.org/13432
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>