Add blur event handling and asynchronous checks to focus fixup rule tests

[lpd-au]

The current focus fixup rule tests seemingly make no attempt to check:

1. If the focus was not changed synchronously, whether the focus was shortly after changed asynchronously
2. If the focus was changed at all, whether a blur event was or was not fired

Focus changes asynchronously

In the past, Chrome would have passed this test asynchronously but failed it synchronously, see crbug.com/660999. Additionally, other implementers have indicated that the timing of focus changes is not straightforward, see w3c/uievents#236 (comment). Therefore, in my opinion, it seems appropriate to test whether a failed synchronous test could still pass asynchronously. I believe this could be worked on immediately.

Event firing

In my opinion, the spec is at best ambiguous whether or not a blur event should be fired, and current browsers have diverging behaviour. To clarify this, I filed whatwg/html#6729. Once this is resolved, I believe we should add tests that verify the accepted behaviour, be that blur firing, not firing or optionally firing.

[lpd-au]

Should be fixed by #38062