message: testing: Simplify staging MPMD test creation
  Convert the tests to `gtest_add_tests_helper`.
diff: find_package(Threads REQUIRED)
  -if(ADIOS2_HAVE_MPI)
  - set(extra_test_args EXEC_WRAPPER ${MPIEXEC_COMMAND})
  -
  - add_executable(TestStagingMPMD TestStagingMPMD.cpp)
  - target_link_libraries(TestStagingMPMD
  - ${CMAKE_THREAD_LIBS_INIT}
  - adios2::thirdparty::gtest adios2::cxx11_mpi MPI::MPI_C
  - )
  +gtest_add_tests_helper(StagingMPMD MPI_ONLY "" Engine.Staging. ".InSituMPI" EXTRA_ARGS "InSituMPI")
  if(ADIOS2_HAVE_SST)
  - gtest_add_tests(TARGET TestStagingMPMD ${extra_test_args}
  - EXTRA_ARGS "SST" "MarshalMethod=FFS"
  - TEST_SUFFIX _SST_FFS
  - TEST_LIST _gtest_added_tests
  - )
  - set_tests_properties(${_gtest_added_tests} PROPERTIES
  - PROCESSORS "${MPIEXEC_MAX_NUMPROCS}"
  - )
  - gtest_add_tests(TARGET TestStagingMPMD ${extra_test_args}
  - EXTRA_ARGS "SST" "MarshalMethod=BP"
  - TEST_SUFFIX _SST_BP
  - TEST_LIST _gtest_added_tests
  - )
  - set_tests_properties(${_gtest_added_tests} PROPERTIES
  - PROCESSORS "${MPIEXEC_MAX_NUMPROCS}"
  - )
  - endif()
  -
  - gtest_add_tests(TARGET TestStagingMPMD ${extra_test_args}
  - EXTRA_ARGS "InSituMPI"
  - TEST_SUFFIX _InSituMPI
  - TEST_LIST _gtest_added_tests
  - )
  - set_tests_properties(${_gtest_added_tests} PROPERTIES
  - PROCESSORS "${MPIEXEC_MAX_NUMPROCS}"
  - )
  + gtest_add_tests_helper(StagingMPMD MPI_ONLY "" Engine.Staging. ".SST.FFS" EXTRA_ARGS "SST" "MarshalMethod=FFS")
  + gtest_add_tests_helper(StagingMPMD MPI_ONLY "" Engine.Staging. ".SST.BP" EXTRA_ARGS "SST" "MarshalMethod=BP")
  endif()
  foreach(helper
  @@ -93,10 +62,6 @@ if(ADIOS2_HAVE_Fortran)
  endif()
  endif()
  -if(ADIOS2_HAVE_MPI)
  - set(extra_test_args EXEC_WRAPPER ${MPIEXEC_COMMAND})
  -endif()
  -
  configure_file(
  run_test.py.gen.in
  ${CMAKE_CURRENT_BINARY_DIR}/run_test.py.gen
  