SwChartDataProvider's XML conversion methods were untested.
Also add tests for SwChartDataSequence's clone method and its get/
set property value methods.

Change-Id: I6323a7af1eae747592f5016b13fb5e2e3f0b71d1
Reviewed-on: https://gerrit.libreoffice.org/26995Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: jan iversen <jani@documentfoundation.org>