KAFKA-20423: Fix flakiness of testWakeupWithFetchDataAvailable

[chickenchickenlove]

Description

I didn't reproduce it in my local.
This PR is based on the specific assumptions and fix non-deterministic path caused by it.

1. The foreground test thread puts the fetch response into MockClient.responses via respondFrom(...).
2. The heartbeat thread enters MockClient.poll() first.
3. The heartbeat thread removes the fetch response from the queue by calling responses.poll().
4. However, it gets descheduled before calling response.onComplete().
5. The foreground thread calls client.poll(0, ...), but since the response queue is empty, it completes nothing and returns.
6. consumer.wakeup() is called.
7. The first consumer.poll(Duration.ZERO) immediately throws WakeupException from maybeTriggerWakeup().
8. consumer.position(tp0) already has a valid position of 0, so it returns 0 directly without performing a network poll.
9. The next consumer.poll(Duration.ZERO) also returns empty because the response is not yet in pendingCompletion and not in the FetchBuffer either.
10. After that, the heartbeat thread wakes up and calls response.onComplete(), but by then the assertion has already
    failed.

Although I didn't reproduce it in my local, this scenario make sense to me.
Given this, this PR applies the change, and I plan to monitor the CI trend afterward to confirm whether the flakiness is resolved.