I'm wondering whether it would be good to increase DUNE_MAX_TEST_CORES from 2 to 4. There are some interesting things that can happen on grids with more than two processes: in particular not every other process must be a direct neighbor, vertices can be part of more processes (e.g. the center in a 2x2 grid). These cases should also be covered by the tests enabled by default, but cannot when parallel tests are only run with 2 ranks.

As an example dune-grid's test-parallel-ug seems to pass with 1 and 2 processes, but currently fails with more (but it isn't run in parallel by default yet).