Unit test for Docx Chart Series Name

UT to verify that the series labels are
imported (or rendered) as in the docx file.

Change-Id: I9e4ade7a905dbb894bc163ecec8c333fa6797818
Reviewed-on: https://gerrit.libreoffice.org/6502
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>