Speed up tests.

tests/test_exchanger_parallel.c

tests/test_idxlist_collection.c

tests/test_idxlist_utils.c

tests/test_idxmod.c

tests/test_idxsection.c

tests/test_idxstripes.c

tests/test_idxvec.c

tests/test_mpi_generate_datatype.c

tests/test_redist_collection_parallel.c

tests/test_redist_collection_static_parallel.c

tests/test_redist_repeat.c

tests/test_redist_repeat_parallel.c

tests/test_request_parallel.c

tests/test_sort.c

tests/test_xmap_common_intercomm_parallel.c

tests/test_xmap_common_parallel.c

tests/test_xmap_intersection_parallel.c