Improve Async Receive Path Testing

[nibanks]

Describe the bug

Testing of the async receive path needs improvement. For one, there's no test to ensure that a slow receiver still works when the sender gracefully shuts the stream down. For instance:

[Sender]
Open unidirectional stream.
Send X bytes.
Shutdown graceful.

[Receiver]
Receive new stream.
Receive data on stream. Return pending.
Wait a little while.
Complete receive (maybe partially?)
Get peer shutdown stream event.
Get shutdown complete stream event.

See #1384 for more details.