Disabling MPI Causes Configuration Error in Unit Test

Although I disable MPI with `cmake -DBUILD_MPI_TESTS:BOOL=OFF -B build .`, 
I get the following error message when I configure the project:

```
CMake Error at test/unit/boost/CMakeLists.txt:13 (target_link_libraries):
  Target "test_boost_exec" links to:

    MPI::MPI_CXX

  but the target was not found.
```