Do not limit number of MPI ranks

debian/dune-debian.mk

@@ -12,6 +12,10 @@ override_dh_auto_configure:
 # OpenMPI looks for a rsh program, even when it only starts local processes.
 export OMPI_MCA_plm_rsh_agent=/bin/false
 
+# Don't limit number of MPI ranks to number of cores (when running tests),
+# see https://bugs.debian.org/850229
+export OMPI_MCA_rmaps_base_oversubscribe=1
+
 override_dh_auto_test:
 	dh_auto_build -- build_tests
 	cd build; PATH=$(CURDIR)/debian/tmp-test:$$PATH $(DUNE_CTEST)