add "short" harness timeout value

[anthumchris]

This would allow tests that explicitly call window.stop() to exit after 2s instead of waiting the full 10s to timeout by adding <meta name="timeout" content="short">. For example, the "short" value could be applied to the tests below:

• https://w3c-test.org/xhr/abort-after-stop.htm
• add test for aborting a document load #9933

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Started: 2018-03-10 18:46:18
Finished: 2018-03-10 18:52:18

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[jgraham]

I'm a bit skeptical of this. I don't think we have any way to know that a test "ought" to time out, so this seems like a workaround for one visible effect of a deeper issue.

[anthumchris]

Yes, I see your point. After more experimentation, explicitly calling window.dispatchEvent(new Event('load')) would resume the natural flow of things after calling window.stop(), and test_harness would complete and avoid the timeout.

That's probably a better solution and I'm happy canceling this PR