Test istlmatrixbackend with a matrix with FM<2,1> blocks

This makes the test run into an assertion failure:

istlbackendtest: /home/sander/dune/dune-fufem/dune/fufem/backends/istlmatrixbackend.hh:106: decltype(auto) Dune::Fufem::Impl::visitMatrixEntryRecursive(Matrix&, const RowIndex&, const ColIndex&, F&&) [with Matrix = Dune::FieldMatrix<double, 2, 1>; RowIndex = Dune::Functions::Imp::ShiftedDynamicMultiIndex<Dune::Functions::StaticMultiIndex<long unsigned int, 1>, 1>; ColIndex = Dune::Functions::Imp::ShiftedDynamicMultiIndex<Dune::Functions::StaticMultiIndex<long unsigned int, 1>, 1>; F = Dune::Fufem::ISTLMatrixBackend<Dune::BCRSMatrix<Dune::FieldMatrix<double, 2, 1>, std::allocator<Dune::FieldMatrix<double, 2, 1> > >, double>::ToScalar<double&>&]: Assertion `iii.size()>0' failed.