private void verifyPreflight() {
        verify(preProducerCheck).run();
        verify(producer).initTransactions();
        verify(postProducerCheck).run();
        verify(offsetStore).start();
    }
@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