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

It is system specific, you're right. But I was just referring that currently it's in `vim.h` without an `#ifdef` for platforms, so it must work reasonably enough.

We should get the situation for platform specific config and defines sorted out.

Perhaps we could make it even better: http://linux.die.net/man/3/realpath (`PATH_MAX` and `NAME_MAX`?)