private void expectApplyTransformationChain() {
        when(transformationChain.apply(any(), any()))
                .thenAnswer(invocation -> invocation.getArgument(1));
    }
@return how many times an {@link OffsetStorageWriter#doFlush(Callback) offset flush} took place
connect/runtime/src/test/java/org/apache/kafka/connect/runtime/ExactlyOnceWorkerSourceTaskTest.java