[RFC] fix unittest issues with busted 2.x

Followup of https://github.com/neovim/neovim/pull/1098#issuecomment-54712118

This is on top of @fwalch's commit because these scoping issues don't occur with busted 1.x.

Regardless of what's going on with busted, it's probably a good thing to avoid global scope. 

ping @aktau because this touches the lua C preprocessor and I'm hoping there's a chance you might have a clue why `popen()` is being weird.

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For reference, this PR should fix the issues noticed by ZyX here: https://github.com/neovim/neovim/issues/903#issuecomment-47494001
I had thought that the buffer tests already called `vim_init` (being that I remember writing `vim_init` for the buffer tests). Must've missed something there.

Good, good. All pass with busted 2 now?