Timout reached in tests

The cmake macro dune_add_test specifies a default timeout (300s). This timeout is reached, e.g., for testpk2dinterpolation in the CI sometimes (depending on the runner). I would suggest, to increase this timeout to something larger, e.g., 600s.

dune_add_test(SOURCES testpk2dinterpolation.cc
              TIMEOUT 600
              COMPILE_DEFINITIONS GRIDSDIR=\"${CMAKE_CURRENT_SOURCE_DIR}/grids\")

see, e.g., https://gitlab.dune-project.org/infrastructure/dune-nightly-test/-/jobs/210845