Level 51
I would suggest that you could create a /widget-sample route that will return from the controller a 'blank'/the default layout page.
The test would look like -
//when I visit that page
//I expect to see empty page/layout
//when I add/inject the widget to the page - as that is what would the user do, it will inject the code into their page.
//I expect to see the title of the widget, some body and some functionality that you know its been written for that widget