Add test for simple-carousel export path

[AndrewJakubowicz]

Completion criteria

In the second BWL video, we install this repo and use the simple carousel. To ensure this behavior doesn't break it may be useful to add a small test that checks that:

npm install https://github.com/lit/video-series-samples.git, followed by an import 'video-series-samples/simple-carousel.js'; registers a simple-carousel to the window.