Need to test asyncFlow with higher fidelity upgrade test

[erights]

See #9125 #9126 #9097 #7966

The point of asyncFlow is for these flows to survive upgrade. However, the current upgrade testing framework asyncFlow uses, as of #9097, is low fidelity. All incarnations co-exist in one JS heap, and their turns may even interleave! This creates known failures to adequately test upgradability #9126 #9125 . Worse, when it succeeds, we don't know what problems might have been detected by a higher fidelity test, giving us less confidence than we'd like.

Most likely, we should follow the precedent of #7966