Run identity test over backbone.js instead of jquery-1.9.1.js.

This makes `whiskey test/identity.js` take 1.4s instead of 3.8s on my machine.
Make require("test/lines").testSlice hog fewer CPU cycles.

Relevant: https://twitter.com/benjamn/status/246397694556372992

This test was always quadratic in the number of characters of code
contained by the testSlice function (and still is!), because I thought
there was some value in being pathologically exhaustive. Nine months
later, I still have not ever needed to fix this test, so I think it's
high time I revisited my original justification.