Bug 1665941 - expand cross-origin requestFullscreen mochitest to test ESC exit. r=smaug

This expands the current requestFullscreen() mochitest to check the
result of exiting fullscreen using the ESC key, since it follows a
separate code path from document.exitFullscreen().

Depends on D90928

Differential Revision: https://phabricator.services.mozilla.com/D90940