Write test for exporting data label positions for percent stack bar chart.

Change-Id: Idebb56fc96828ec38d5c239c16e5b413a50c837f
Ensure we export correct labal placement value for percent/stacked charts.

Normal charts allow a variety of label placement options, but percent/stacked
charts only allow three variants, and exporting a wrong value would trigger
MS Office to think the file is corrupt.

Change-Id: I8bdc1dc072b29e8df2c506b6b16c61279df12045