Fixes apache#24788

Motivation

The test is flaky. Beacuse:
In this test, the TransactionBufferTestImpl class extends TopicTransactionBuffer and declares a field with the same name publishFuture.
Therefore, in the appendBufferToTxn method, getPublishFuture() retrieves the publishFuture from the subclass, while the assignment publishFuture = future modifies the publishFuture belonging to the parent class.

 CompletableFuture<Position> future = getPublishFuture().thenCompose(ignore -> {
            ......
        }).whenComplete(((position, throwable) -> buffer.release()));
publishFuture = future;

Modifications

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