cmake: allow to oversubscribe the machine during MPI tests

Beginning with version 3.0, OpenMPI's mpirun refuses to start
more processes than there are physical processor cores on the
machine.

To allow oversubcribing, the 'OMPI_MCA_rmaps_base_oversubscribe'
environment variable can be set to 1.

test/CMakeLists.txt

@@ -159,6 +159,11 @@ function(addTest TEST_NAME TEST_SOURCES TEST_LIBRARIES WORKING_DIRECTORY N_PROCS
     # Add test
     add_test(NAME ${TEST_NAME} COMMAND ${TEST_EXEC} ${TEST_ARGS} WORKING_DIRECTORY "${WORKING_DIRECTORY}")
 
+    # Set test variables
+    if (${N_PROCS} GREATER 1)
+        set_tests_properties(${TEST_NAME} PROPERTIES ENVIRONMENT OMPI_MCA_rmaps_base_oversubscribe=1)
+    endif()
+
 endfunction()
 
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