Solve OpenMPI binding issue

The OpenMPI -oversubscribe feature was backported to the 2.X series.
Without the -oversubscribe flag, tests running on hyperthreaded CPUs
with more threads than cores using OpenMPI 2.X may exit early with
an exit code of 0, causing these tests to silently fail.

cmake/unit_test.cmake

@@ -5,9 +5,9 @@ if(NOT DEFINED TEST_NP)
 endif()
 
 if(EXISTS ${MPIEXEC})
-  # OpenMPI 3.0 and higher checks the number of processes against the number of CPUs
+  # OpenMPI 2.0 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 ERROR_VARIABLE mpi_version_output)
-  if (mpi_version_result EQUAL 0 AND mpi_version_output MATCHES "\\(Open(RTE| MPI)\\) ([3-9]\\.|1[0-9])")
+  if (mpi_version_result EQUAL 0 AND mpi_version_output MATCHES "\\(Open(RTE| MPI)\\) ([2-9]\\.|1[0-9])")
     set(MPIEXEC_OVERSUBSCRIBE "-oversubscribe")
   else()
     set(MPIEXEC_OVERSUBSCRIBE "")