More fine-grained check of data series labels in the unit test.

Change-Id: I6b23126816bb114407f2b75545094bc297ee7cc3
(cherry picked from commit bdad8d9051c207b2eff0d5a7842da83e6119a600)
Reviewed-on: https://gerrit.libreoffice.org/10041
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>