consumer: return from Poll if the only error is an injected fake

Poll previously would not return if only a fake fetch existed. For
example, if a user joined a group with an instance ID twice, the fenced
group member would never exit polling. We now check both the actual
ready sources and the fake sources in our poll condition loop.

Closes #479.

pkg/kgo/consumer.go

@@ -486,7 +486,7 @@ func (cl *Client) PollRecords(ctx context.Context, maxPollRecords int) Fetches {
 		defer c.sourcesReadyMu.Unlock()
 		defer close(done)
 
-		for !quit && len(c.sourcesReadyForDraining) == 0 {
+		for !quit && len(c.sourcesReadyForDraining) == 0 && len(c.fakeReadyForDraining) == 0 {
 			c.sourcesReadyCond.Wait()
 		}
 	}()