dune_add_test: MPI_RANKS / DUNE_MAX_TEST_CORES
MPI_RANKS greater than
DUNE_MAX_TEST_CORES are silently omitted.
Furthermore there is no advertising that the option
DUNE_MAX_TEST_CORES exists or what the default is.
MPI_RANKS 1 3 4 for example will silently result in no parallel test, since the default for
DUNE_MAX_TEST_CORES is 2.
I would be nice if at least a warning and a hint to
DUNE_MAX_TEST_CORES would be emitted