Bug#966965: dune-grid-glue: FTBFS: terminate called after throwing an instance of 'Dune::Exception'

[Ansgar]

Control: tag -1 + upstream

Lucas Nussbaum writes:
> Source: dune-grid-glue
> Version: 2.7.0-2
>>   what():  Dune::Exception [eh:/<<PKGBUILDDIR>>/dune/grid-glue/test/ringcommtest.cc:24]: MPI ERROR -- MPI_ERR_TAG: invalid tag
>>   what():  Dune::Exception [eh:/<<PKGBUILDDIR>>/dune/grid-glue/test/ringcommtest.cc:24]: MPI ERROR -- MPI_ERR_TAG: invalid tag

The is probably caused by `int tag = 12345678;` in the code while
OpenMPI might have a lower limit, see [1].

  [1]: https://github.com/open-mpi/ompi/issues/6940

Ansgar