message: testing: Add both Serial and MPI tests of SST engine
  Convert sst engine tests to `gtest_add_tests_helper` to take advantage
  of the infrastructure from commit (testing: Teach test helper
  functions to add both Serial and MPI tests, 2020-04-30).
diff: # accompanying file Copyright.txt for details.
  #------------------------------------------------------------------------------#
  -if(ADIOS2_HAVE_MPI)
  - set(extra_test_args EXEC_WRAPPER ${MPIEXEC_COMMAND})
  -endif()
  -
  -add_executable(TestSstParamFails TestSstParamFails.cpp)
  -target_link_libraries(TestSstParamFails adios2::thirdparty::gtest)
  -gtest_add_tests(TARGET TestSstParamFails ${extra_test_args})
  -
  -add_executable(TestSstWriterFails TestSstWriterFails.cpp)
  -target_link_libraries(TestSstWriterFails adios2::thirdparty::gtest)
  -gtest_add_tests(TARGET TestSstWriterFails ${extra_test_args})
  -
  -if(ADIOS2_HAVE_MPI)
  - target_link_libraries(TestSstParamFails adios2::cxx11_mpi MPI::MPI_C)
  - target_link_libraries(TestSstWriterFails adios2::cxx11_mpi MPI::MPI_C)
  -else()
  - target_link_libraries(TestSstParamFails adios2::cxx11)
  - target_link_libraries(TestSstWriterFails adios2::cxx11)
  -endif()
  +gtest_add_tests_helper(SstParamFails MPI_ALLOW "" Engine.SST. "")
  +gtest_add_tests_helper(SstWriterFails MPI_ALLOW "" Engine.SST. "")
  