dune-ctest does not like non-ASCII test output

The ARPACK test in dune-istl produces non-ASCII output (in particular, a λ), which dune-ctest doesn't like (cf. https://gitlab.dune-project.org/core/dune-istl/-/jobs/42887, it complains about ordinal not in ASCII range). This confuses me a little because we use latin-1 for reading in the test output. @ansgar: Can we just change that codec to utf-8, or might that break other stuff?