Tests should use CMAKE_GUARD to show up as skipped.

Many tests in ISTL are inside some condition, e.g.

if(cond)
  dune_add_test(...)
endif()

Instead, they should make use of CMAKE_GUARD:

dune_add_test(... CMAKE_GUARD cond)

This would have the benefit that those tests will show up as skipped so you know that you're not testing everything.