Make tests work with OpenMPI 3.0

OpenMPI 3.0 by default does not allow running with more processes than CPUs

CMakeLists.txt

@@ -810,6 +810,12 @@ if ( WALBERLA_BUILD_WITH_MPI AND NOT WALBERLA_CXX_COMPILER_IS_MPI_WRAPPER )
      endif ( )
    endif ( )
 endif ( )
+
+# OpenMPI 3.0 and higher checks the number of processes against the number of CPUs
+execute_process(COMMAND ${MPIEXEC} --version RESULT_VARIABLE mpi_version_result OUTPUT_VARIABLE mpi_version_output)
+if (mpi_version_result EQUAL 0 AND mpi_version_output MATCHES "\\(Open(RTE| MPI)\\) ([3-9]\\.|1[0-9])")
+   set ( MPIEXEC_PREFLAGS "${MPIEXEC_PREFLAGS} -oversubscribe" )
+endif()
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