test(cupertino_http): Retry cancellation tests.

[brianquinlan]

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[natebosch]

Is it feasible to coordinate the sequencing without an arbitrary delay? Should we wait on a Completer here and have the test body complete it after the cancellation runs? It will require edits in existing tests, but much better to be explicit about the interleaving.

[brianquinlan]

I misunderstood what was going on. I removed the delay and updated the documentation.

What is happening is that that cancel, when complete, transitions to the completed state. And that sometimes happens before we can observe the state as cancelled.

If this doesn't make the test reliable, I should probably just remove it.