test(pubsub): make error checking of receive less brittle

[hongalex]

Receive can occasionally return context.Canceled. This makes tests that call Receive less brittle by checking explicitly for canceled. This also replaces checks of the codes.Canceled status code.

[shollyman]

Generally, these all seem like a good case for a swap to errors.Is(err, context.Cancelled) rather than a direct equality check.

[shollyman]

approving based on discussed refactoring