Solved by adding additional shape filling whole diagram.
MS PowerPoint does the same when converting SmartArt to shapes.

Background shape is also copied when loading from drawingML fallback,
appearently there is no background information.

Corrected SmartArt import tests, so that they are aware of extra shape.

Change-Id: I6154f8e1b34e5867ab582d6fc54459c7c93edbac
Reviewed-on: https://gerrit.libreoffice.org/72012
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>