Add tests for async_channel

[amab8901]

(follow-up on #837)

This addresses the last check on: #715

The following scenarios should be tested:

• the clone method on Sender/Receiver
• dropping a Sender or Receiver when multiple ones exist already, rather than just a single one.
• Trying to send to or receive from a disconnected channel
• Whether the Receiver::sender() and Sender::receiver() methods properly change the Channel's state in the case of multiple endpoints or a single endpoint.
• Actually recovering from a channel disconnection using the above methods and then successfully sending and/or receiving through the channel
• Anything with more than two tasks -- a common use case would be to have several sending tasks and several receiving tasks, so we want to test whether that works properly.

But first, I think we need to identify every scenario that should be tested, so that we aren't missing anything

[kevinaboos]

Logistics comment: rather than opening up duplicate issues, can you merge them into one? Might as well add these checkbox bullet points to #715.