This suite is intended to test the behavior of `Chart#layer`--that it modifies the provided `d3.selection` instance as we expect.

This new test on the other hand verifies that the `Chart` prototype defines an `unlayer` method. I don't think that "shape" tests are really that useful (the tests in `intergration.js` will fail with the relatively straightforward message "Undefined is not a function" in the event that we somehow accidentally remove the entire method), so I recommend removing this.