Add test suite

It's probably hard to test the actual core feature of `react-perimeter` , but there are a few things we can definitely test:

* correctly exports the React component
* renders children
* attaches `ref` correctly with render callbacks
* registers event listeners on mount
* removes event listeners on unmount
* removes event listeners after `onBreach` is called if `once` is `true`
* more stuff

I'd like to use `jest` and `enzyme` for this.

Start by locating the react-perimeter React component and checking whether a test setup already exists. Use the listed export, rendering, ref, listener, unmount, and once/onBreach behaviors as test cases; done means the suite covers these behaviors and passes.

Written by the indexing model from the issue text.