Add tests for localStorage backend

Oh, we should have these! So I wrote some!

The plan will be to add more tests, as well as support for WebSQL driver (via CasperJS natively) and IndexedDB driver (using [SlimerJS](http://slimerjs.org/)).

It's based off my tests in [Face Value](https://github.com/tofumatt/face-value/tree/master/test) and some of the work would likely get backported into [sisyphus](https://github.com/tofumatt/sisyphus), my ignored-but-possibly-promising headless browser testing harness.