[cmake] ISTL uses FindSuiteSparse.cmake
the FindSuiteSparse.cmake is part of dune-common, but it is used in istl and not common. As common provides the check, I think it should also run the check. Then istl can use use the results. If the tests should be run by istl, I'd expect that the tests are part of istl.
The thing that is particularly strange is that cmake/modules/DuneIstlMacros.cmake explicitly includes AddSuiteSparseFlags, which is not part of istl.