ToAsyncEnumerable_Observable_Cancel_InFlight sometimes fails spuriously

[idg10]

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.