Write tests for AOM User Action Events

[cookiecrook]

Once there is a way to trigger/synthesize AT events in WPT (maybe via AOM #203), we should write tests for AOM's "User Action Events." (Partially shipping in WebKit but not others yet.)

https://github.com/WICG/aom/blob/gh-pages/explainer.md#user-action-events-from-assistive-technology

In-page explainer links are fragile, so in case that link doesn't work by the time someone gets to it, search the page for "user action." There are 4 tables worth of expectations that need testing re: the event types and expected event object properties.

https://github.com/WICG/aom/blob/gh-pages/explainer.md