During export of the TOC into DOCX all TOC field parts
should be placed inside one STD frame: field start-begin,
field-separate and field-end mark.

Updated unit test.

Change-Id: I10e37a48160e1dc4f0074c4e2e7621a59f79c2ee
Reviewed-on: https://gerrit.libreoffice.org/66587
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>