GeometryGrid: make tests for fake entities pass

test-geogrid.cc failed when built with UG as the host grid: the implementation of the CommDataHandle called a constructor only available for real entities with only a runtime check of the codimension. This merge request adds the missing constructor to the interface of fake entities, although it can only throw an exception if invoked.

In addition a wrong assertion that assumed fake entities are convertable to bool was dropped.