Require non-trivial distribution only for recent Dune

The `CpGrid::scatterGrid()` method produces non-trivial results only if we have MPI support **and** if the target Dune platform is sufficiently recent (i.e., dune-grid >= 2.3).  Therefore, we cannot "REQUIRE" non-trivial results unless both conditions are satisfied.

In the process, also extend the diagnostic in `CpGrid::scatterGrid()` to include information about the dune-grid requirement.

This fixes "make test" in configurations with MPI and Dune 2.2.1.