Write test for doughnut chart export of data labels to OOXML.

We must not export label position properties for doughnut charts, else
MS Office would think the file is corrupt.

Change-Id: Icfab257ffb6da945b8add661e6455aa66cc475ab
Doughnut charts don't support label placement option. Don't export it.

Change-Id: I6d0e2c099869120bdf594813468a3c5ba4bb46fd
Add test for default data label placement for line chart.

Change-Id: I25cf48703a286470907ccc5415a0fab479aa4b8c
Default data label placement may vary depending on chart types.  Get it right.

If we export a wrong placement value, MS Office will flag the file corrupt and
the loading will fail.

Change-Id: I7ca1239cd390494c1181ecdb3310c5f88bb18f9b