BUG: Fix vtkMRMLSceneViewNodeStoreSceneTest randomly failing on windows

Allocating a single "vtkMRMLScalarVolumeNode" wasn't enough to ensure
that the space previously used by 'vtkMRMLScalarVolumeNode1' wasn't
available again for the restored volume.

See inlined comment for more details.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22594 3bd1e089-480b-0410-8dfb-8563597acbee