Disable some widget tests building for OpenGL2

A few of the volume rendering classes are currently excluded when
building the OpenGL2 backend, these should be restored once this
changes.

Change-Id: I35fcb41313639c23c08033ffa6d19703430d1e37
Merge topic 'widget-volume-gl2' into master

9b17ebdd Disable some widget tests building for OpenGL2