Add more diagnostic tests and error notifications to test page

[amandabaker]

The plan is to add more diagnostics to https://amandabaker.github.io/pwa/windowControlsOverlay/index.html

Scenarios to consider:

• Do clicks/events pass through the WCO to the web content?
• Is the app successfully registering for the geometrychange event
• Are the CSS environment variables being set?
• Is the geometrychange event firing?
• Does the draggable region work? (not sure how to test this one though...)