Fix MPI_Finalize Problem

- MPI_Finalized(&flag) tells us if MPI finalized already ^^
- Parallel_SimpleDataTest.cpp does not compile due to an
  undefined call to `memset`
Disable Parallel_SimpleDataTest

```
libSplash/tests/Parallel_SimpleDataTest.cpp:
  In member function ‘bool Parallel_SimpleDataTest::subtestWriteRead(int32_t,
  int, DCollector::Dimensions, DCollector::Dimensions, DCollector::Dimensions,
  uint32_t, MPI_Comm)’:
libSplash/tests/Parallel_SimpleDataTest.cpp:143:68:
  error: ‘memset’ was not declared in this scope

libSplash/tests/Parallel_SimpleDataTest.cpp:
  In member function ‘bool Parallel_SimpleDataTest::subtestFill(int32_t, int,
  DCollector::Dimensions, DCollector::Dimensions, uint32_t, MPI_Comm)’:
libSplash/tests/Parallel_SimpleDataTest.cpp:396:72:
  error: ‘memset’ was not declared in this scope
```