MPI test failure: invalid tag

A rebuild in Debian triggered an exception:

what(): Dune::Exception [eh:/<>/dune/grid-glue/test/ringcommtest.cc:24]: MPI ERROR -- MPI_ERR_TAG: invalid tag

This is probably as int tag = 12345678; is used in the code while OpenMPI might have a lower limit. Maybe just use int tag = 0;? That should be good for ~32k processes as tag + rank would then stay under 32k, the minimum upper bound for valid tags.

I've assigned to bug to @christi as he seems to use that part of dune-grid-glue and worked on it.