message: enable test with mpi oversubscribe
  OMPI_MCA_rmaps_base_oversubscribe=yes must be passed to cmake or be an
  env variable
diff: @@ -134,8 +134,14 @@ if(ADIOS2_HAVE_MPI)
  cmake_host_system_information(RESULT ncores QUERY NUMBER_OF_PHYSICAL_CORES)
  message("-- Found MPI nprocs = " ${ncores})
  if(ncores LESS 4)
  + if (NOT "$ENV{OMPI_MCA_rmaps_base_oversubscribe}" STREQUAL "yes")
  message("-- Reducing MPI nprocs to " ${ncores} " for tests")
  - set(MPIEXEC_MAX_NUMPROCS ${NPROCS})
  + set(MPIEXEC_MAX_NUMPROCS ${ncores})
  + else()
  + message("-- Found OMPI_MCA_rmaps_base_oversubscribe")
  + set(MPIEXEC_MAX_NUMPROCS 4)
  + message("-- Setting MPI nprocs = " ${MPIEXEC_MAX_NUMPROCS})
  + endif()
  endif()
  # Workaround for MPI forcing the link of C++ bindings
  add_definitions(-DOMPI_SKIP_MPICXX -DMPICH_SKIP_MPICXX)
  