ToAsyncEnumerable_Observable_Cancel_InFlight sometimes fails spuriously

We occasionally get failures in the `ToAsyncEnumerable_Observable_Cancel_InFlight` test. When we rerun the build it typically succeeds.

It's possible that this is because there are two different phases in which cancellation could occur—the initial `await` when we acquire the enumerator, and then the part the test inspects, the `MoveNextAsync`. Or it's also possible that it's related to the fact that we're using `ValueTask` in a slightly unorthodox way. Or it could be something else. We need to investigate.