[RFC] refactor includes + improve testing infrastructure

This will allows testing of more files and allow PR #904 to proceed.

Implements a tiny part of issue #918 in the process as well.
Yes! This must have been for C89 and/or older MSVC versions. [msinttypes.h](http://msinttypes.googlecode.com/svn/trunk/stdint.h) defines `uintptr_t` exactly Vim does (except for win32 it uses `int` instead of `long`)