fix(setTimeout): allow clearTimeout to be called in setTimeout callback

[frankwallis]

This is a fix for #290 - "More tasks executed than were scheduled" error.

The change is to set task.cancelFn = null before invoking a task for non-periodic tasks. As it is no longer possible to cancel the task which is just about to be invoked, I think this should be ok. I also updated the setTimeout tests to create new zones to run the tests in, as initially the test was incorrectly passing due to the task counts not being 0 to begin with.

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[frankwallis]

I have signed the CLA

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