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

There's a good discussion here, with some conflicting opinions: http://stackoverflow.com/a/1674459/152142 . But see also [c faq](http://c-faq.com/ansi/constasconst.html). The way we are currently accessing `#define` values is to provide them to the lua preprocessor like this: https://github.com/neovim/neovim/blob/master/test/includes/pre/sys/fcntl.h