[test] Use FoamGrid for 2d in 3d grid as alu doesn't support networks just surfaces

Fixes #567 (closed).

dune-alugrid only supports surface not network grids. The resulting vtk file was still correct as it doesn't care about face connectivity. But the assert signified that something is wrong. Now we use dune-foamgrid, the grid was also made with dune-foamgrid.