[WIP/RFC] Add unit tests for buffer.c and fileio.c

The `buffer.moon` file segfaults. I do not understand why. Any pointers would be appreciated.
I ran into an issue because `p_cpo` wasn't set during my tests. I saw somewhere else checked `p_cpo != NULL` so I added that here. If that is inappropriate, I need to figure out how to initialize `p_cpo` properly.