[RFC] fix unittest issues with busted 2.x

Marking this RFC because other than cleaning up the commit messages (and squashing), I don't plan to do much else.

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. 

There's also some other weird issues that came up that I'm not sure are related to busted: they could be caused by other included packages such as penlight (I didn't investigate this though).

ping @fwalch @tarruda 

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.
I actually have the same hack in a personal branch, but never bothered to submit it because it "vanished" after some Lua-C preprocessor fixes long ago. Also I was ashamed that I couldn't find the actual issue behind this. Sometimes the body is nil indeed...