ornladios#725 enable test with mpi oversubscribe

OMPI_MCA_rmaps_base_oversubscribe=yes must be passed to cmake or be an
env variable

CMakeLists.txt

@@ -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)
-    message("-- Reducing MPI nprocs to " ${ncores} " for tests")
-    set(MPIEXEC_MAX_NUMPROCS ${NPROCS})
+    if (NOT "$ENV{OMPI_MCA_rmaps_base_oversubscribe}" STREQUAL "yes")
+      message("-- Reducing MPI nprocs to " ${ncores} " for tests")
+      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)