To fully exit fullscreen is not fully tested / testable

[foolip]

The Fullscreen API currently has a fully exit fullscreen algorithm and in whatwg/fullscreen#65 this is being changed into a fullyFlag that achieves the same effect.

There are the cases where it's invoked:

• "Whenever the unloading document cleanup steps run with a document" – could be tested by opening a separate window, going fullscreen in that window, and then navigating it.
• In the exit fullscreen steps, "If resize is true and topLevelDoc is either not in exitDocs, or not a simple fullscreen document" – can only happen with racing enter+exit requests, and this is not well defined, see Decide on wanted behavior for request+exit or exit+request together whatwg/fullscreen#63 and Test the many ways for fullscreen requests and exits to compete #4250
• "If the end user instructs the user agent to end a fullscreen session initiated via requestFullscreen()" – untestable even with input automation because the key binding or gesture is UA-defined.

whatwg/fullscreen#70 is something that would allow the mechanism itself to be tested, but it could not confirm that the three existing entry points are implemented.