fix: make tests compatible with Chrome/ium

Author: rugk

tests/setup.js

@@ -1,4 +1,5 @@
 import "https://unpkg.com/mocha@5.2.0/mocha.js"; /* globals mocha */
+import "https://unpkg.com/webextension-polyfill@0.8.0/dist/browser-polyfill.js"
 // import "./node_modules/sinon/pkg/sinon.js";
 
 mocha.setup("bdd");