message: testing: Simplify MPI-only Test.Engine.Common cmake logic
  This test directory is only added when `ADIOS2_HAVE_MPI` is true.
  Drop explicit tests of that condition inside inside the directory.
diff: find_package(Threads REQUIRED)
  add_executable(Test.Engine.Common TestEngineCommon.cpp)
  -if(ADIOS2_HAVE_MPI)
  - target_link_libraries(Test.Engine.Common gtest_interface adios2::cxx11_mpi MPI::MPI_C)
  -else()
  - target_link_libraries(Test.Engine.Common adios2::cxx11)
  -endif()
  -target_link_libraries(Test.Engine.Common gtest ${CMAKE_THREAD_LIBS_INIT})
  +target_link_libraries(Test.Engine.Common PRIVATE
  + gtest_interface
  + adios2::cxx11_mpi
  + MPI::MPI_C
  + gtest
  + ${CMAKE_THREAD_LIBS_INIT}
  +)
  # Note: extra arguments
  # 1st arg: Engine
  # 2nd arg: 1 for serialized execution, 0 for concurrent execution of Writer/Reader
  # 3rd arg: engine parameters
  -gtest_add_tests_helper(Common ${test_mpi} "" Engine. .File
  +gtest_add_tests_helper(Common TRUE "" Engine. .File
  EXTRA_ARGS "File" "1"
  )
  if(ADIOS2_HAVE_HDF5 AND HDF5_IS_PARALLEL)
  - gtest_add_tests_helper(Common ${test_mpi} "" Engine. .HDF5
  + gtest_add_tests_helper(Common TRUE "" Engine. .HDF5
  EXTRA_ARGS "HDF5" "1"
  )
  endif()
  if(ADIOS2_HAVE_SST)
  - gtest_add_tests_helper(Common ${test_mpi} "" Engine. .SST.FFS
  + gtest_add_tests_helper(Common TRUE "" Engine. .SST.FFS
  EXTRA_ARGS "SST" "0" "MarshalMethod:FFS"
  )
  - gtest_add_tests_helper(Common ${test_mpi} "" Engine. .SST.BP
  + gtest_add_tests_helper(Common TRUE "" Engine. .SST.BP
  EXTRA_ARGS "SST" "0" "MarshalMethod:BP"
  )
  endif()
  -if(ADIOS2_HAVE_MPI)
  gtest_add_tests_helper(Common TRUE "" Engine. .InSituMPI
  EXTRA_ARGS "InSituMPI" "0"
  )
  -endif()
  #if(ADIOS2_HAVE_DataMan)
  # gtest_add_tests_helper(Common TRUE "" Engine. .DataMan
  