Add test for the `Beacon API`

* W3C Specification: https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/Beacon/Overview.html
* Support for this API has [recently landed in `Blink`](https://codereview.chromium.org/232053005/), and it's already present in the `Firefox Nightly Build`:

  ![](https://cloud.githubusercontent.com/assets/1223565/2928473/d26b9344-d77d-11e3-9ce7-c973ac70a74d.png)