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

This is required so that the constants are available. It prevents the build from creating the file though.

If I add it manually to the generated file in `build/test/includes/post/nvim/buffer.h` then the test runs correctly.
As it currently is, the build fails.

Is there a better way to be doing this?

Also the build did not pick up that I had added this file initially. Eventually it did, though it seemed to have been triggered by me messing around in `test/includes/CMakeLists.txt` which maybe made the build think that the file could be stale.