Write tests for Owl

[CptFoobar]

As Owl has grown over the years, I now feel the need to have a testing framework in place that would verify Owl's functionality before I push updates. The recent v2.1.x fiasco makes this evident. The problem is, I have no experience with JS testing frameworks, and there doesn't seem to be an out-of-the-box testing framework for WebExtensions anyway. However, I found the following links that may be of relevance:

• https://github.com/mdn/webextensions-examples/tree/master/mocha-client-tests
• https://github.com/Standard8/example-webextension
• https://github.com/mozilla/web-ext/issues/497

The tests to be written are a topic for discussion for this thread.