Add a test for unexpected activation triggered during space keydown driven focus management

[muan]

Relevant spec: https://w3c.github.io/uievents/#event-flow-activation

This is targeting a Firefox bug where when forwarding focus on space and enter keydown events, a unexpected click event is triggered on the forward target.

Here's a demo page for the buggy behavior: https://html-is.glitch.me/firefox-space-focus.html

Since this is my first WPT pull request, I want to keep it small– this test case only cover the Space key case where Firefox is the outlier.

There might be a better place for this test file. I'd be happy to take any suggestions and make changes accordingly.

Many thanks to @Hexcles & @jgraham for helping me in IRC.

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Reference re \ue00d: https://bugzilla.mozilla.org/show_bug.cgi?id=1417955

[Hexcles]

Is this safe to land now?

[muan]

@Hexcles Not sure who this question is for, but for what it's worth I've done all I can here. Is there something this needs to wait for?

There is this new comment on https://bugzilla.mozilla.org/show_bug.cgi?id=1220143#c12, which I've replied to.

[Hexcles]

Thanks, @muan. I'm merging this now. If anyone (including folks on the Bugzilla thread) has further comments, we can always follow up.