[!342] Adjust to latest dune-testtools

Merge branch 'feature/latest-dune-testtools' into 'master'

ref:extensions/dune-codegen Which changed the interfacing between Python and
CMake.

See merge request [extensions/dune-codegen!342]

  [extensions/dune-codegen!342]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/342