[!222] [matrixtest.cc] Enable test for 1x1 FieldMatrices.

Merge branch 'matrixtest-1x1-interface' into 'master'

Found by clang 6 -Wunused-variable.

No idea why the FieldMatrix is tested in ISTL, but whatever.

See merge request [!222]

  [!222]: gitlab.dune-project.org/core/dune-istl/merge_requests/222