Improve testing

[conorgil]

We have ignored automated testing so far because < insert unconvincing reason here >. Testing is critical because < insert a bajillion reasons here > and we should do better.

• write a document listing the specific steps to test the extension before publishing an updated version. This will all be manual, but having a list reduces the chance of skipping/forgetting something. Step in the right direction.
• research the best way to write automated tests for a web extension and then do those things