gh-111065: Add more tests for the C API with the PySys_ prefix (GH-111067)

* Move existing tests for PySys_GetObject() and PySys_SetObject() into
  specialized files.
* Add test for PySys_GetXOptions() using _testcapi.
* Add tests for PySys_FormatStdout(), PySys_FormatStderr(),
  PySys_WriteStdout() and PySys_WriteStderr() using ctypes.