test: run browser script tests in headless Chrome

The tests for the browser scripts were previously run in a virtual
DOM (within Jest’s JDOM environment). This commit introduces a new
Jest environment (and companion utils) to run tests in a full browser
environment (headless Chrome, driven by Puppeteer).

Jest’s mock facility is available in the browser context through the
`window.mock` object.

package.json

@@ -80,7 +80,7 @@
     "eslint": "^3.19.0",
     "eslint-config-airbnb-base": "^11.2.0",
     "eslint-plugin-import": "^2.3.0",
-    "jest": "^21.1.0",
+    "jest": "21.3.0-beta.3",
     "rimraf": "^2.6.1",
     "standard-version": "^4.2.0",
     "uglify-js": "^3.0.8",