Kafka MessageDrivenAdapterTests: batch race condition

The `ConsumerConfig.FETCH_MIN_BYTES_CONFIG` is `1` byte by default.
Therefore, the batch consumer expectations might be fulfilled too early
just for one record.
Adjust such a config to `12` bytes to be sure that both published records
are received in a single batch: exactly what we expect in the test logic.

**Auto-cherry-pick to `6.4.x`**

Author: artembilan

spring-integration-kafka/src/test/java/org/springframework/integration/kafka/inbound/MessageDrivenAdapterTests.java

@@ -398,6 +398,8 @@ protected boolean doSend(Message<?> message, long timeout) {
 	void testInboundBatch() throws Exception {
 		Map<String, Object> props = KafkaTestUtils.consumerProps(EMBEDDED_BROKERS, "test2", "true");
 		props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");
+		props.put(ConsumerConfig.FETCH_MIN_BYTES_CONFIG, 12);
+
 		DefaultKafkaConsumerFactory<Integer, String> cf = new DefaultKafkaConsumerFactory<>(props);
 		ContainerProperties containerProps = new ContainerProperties(topic2);
 		containerProps.setIdleEventInterval(100L);