DRY tests, plus test for var decl w/o assignment

Added `var foo;` to `define-simplified-wrapper`. Declaring a variable without assigning it is a trick used to speed up determining whether or not a variable is undefined without having to let the JS engine probe all the way to the global scope. It's an optimization that only makes sense in loops with many iterations. `browserify-ftw` was failing on this. 

While I was at it, I refactored some of the tests to DRY them up.

This fixes https://github.com/thlorenz/browserify-ftw/issues/5