[!247] [cleanup] Cleanup dotproducttest.cc

Merge branch 'feature/cleanup-dotproducttest' into 'master'

ref:core/dune-istl

-   Use Dune::Std::negation to negate meta function
-   Indent Dune::Hybrid::ifElse branch for better readability

See merge request [core/dune-istl!247]

  [core/dune-istl!247]: gitlab.dune-project.org/core/dune-istl/merge_requests/247