[bugfix][tests] Fix grid construction for tests with dune-alugrid
The way of constructing simple unitsqure-type grids in gridexamples.hh seems outdated. This patch uses StructuredGridFactory instead, to enable actual testing with dune-alugrid.
The way of constructing simple unitsqure-type grids in gridexamples.hh seems outdated. This patch uses StructuredGridFactory instead, to enable actual testing with dune-alugrid.