Testing can't run without MPI

[Cstandardlib]

Describe the Testing Issue

I want to do some testing without MPI.
However, there is one parallel test that can not run without MPI. Should some flag be added to ensure it will only compile when MPI is ON?

CMake Error at CMakeLists.txt:661 (add_executable):
  Target "hsolver_parak2d_test" links to target "MPI::MPI_CXX" but the target
  was not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?
Call Stack (most recent call first):
  source/module_hsolver/test/CMakeLists.txt:153 (AddTest)

Also, some tests do not wrap MPI-related headers and functions in __MPI macros. I'll fix them.

Additional Context

No response

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