[operators] use default la backend from dune-stuff
[tests.elliptic-...-discretization] use default la backend from dune-stuff
[tests.elliptic-swipdg-discretization] minor updated
[tests.elliptic-testcases] updated grid creation

Recently the Grid::Providers::Cube in dune-stuff was updated to do one
refinement of the initial grid in case of the conforming alugrid. Thus we
have to remove one refinement level here...