HOTFIX: testInitTransactionTimeout should use prepareResponse instead of respond

[abbccdda]

We have seen a flaky behavior due to using #respond instead of #prepareResponse call for the txn test.

Reproduced on local as:

[2020-02-26 15:08:49,194] ERROR [Producer clientId=producer-bad-transaction, transactionalId=bad-transaction] Uncaught error in kafka producer I/O thread:  (org.apache.kafka.clients.producer.internals.Sender:241)
java.lang.ClassCastException: org.apache.kafka.common.requests.InitProducerIdResponse cannot be cast to org.apache.kafka.common.requests.FindCoordinatorResponse
	at org.apache.kafka.clients.producer.internals.TransactionManager$FindCoordinatorHandler.handleResponse(TransactionManager.java:1494)
	at org.apache.kafka.clients.producer.internals.TransactionManager$TxnRequestHandler.onComplete(TransactionManager.java:1260)
	at org.apache.kafka.clients.ClientResponse.onComplete(ClientResponse.java:109)
	at org.apache.kafka.clients.MockClient.poll(MockClient.java:294)
	at org.apache.kafka.clients.producer.internals.Sender.maybeSendAndPollTransactionalRequest(Sender.java:458)
	at org.apache.kafka.clients.producer.internals.Sender.runOnce(Sender.java:312)
	at org.apache.kafka.clients.producer.internals.Sender.run(Sender.java:239)

and

java.lang.IllegalStateException: No requests pending for inbound response FindCoordinatorResponseData(throttleTimeMs=0, errorCode=0, errorMessage='NONE', nodeId=0, host='host1', port=1000)

	at org.apache.kafka.clients.MockClient.respond(MockClient.java:345)
	at org.apache.kafka.clients.MockClient.respond(MockClient.java:319)
	at org.apache.kafka.clients.producer.KafkaProducerTest.testInitTransactionTimeout(KafkaProducerTest.java:770)

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[ijuma]

ok to test

[guozhangwang]

@abbccdda Verified on local this could work. Merging to trunk now.