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Doesn't look like you test or verify anything in your readingInvalidFileAndMoveItToErrorDir() at all. The fileReadingEndpoint.start(); is the last line of the test method.

Please, consider to investigate what is JUnit and how we should write test methods and what we should do to really verify async solutions like integration flow similar to yours: https://junit.org/junit5/docs/current/user-guide/

What I see so far is not good. The message source is about polling with some period. Your mockInvalidStudentFile is about producing the same file all the time. Is it really expected in your solution? You probably can just consider to send your file direct to the channel the message source is configured for.

You don't need a fileReadingEndpoint.start(); since substituteMessageSourceFor() does autostart up by default.

Your message after sending to the channel with the file as a payload is going to travel through the flow. You probably should see how you would verify the correctness of your logic in the test. Since you say that you drop the file in the end of the flow in some dir, so probably you should check that dir after sending. Maybe even using some loop or Awaitility: https://github.com/awaitility/awaitility.

Another way is to use a substituteMessageHandlerFor() so you would place a MockMessageHandler instead of your file writer to verify the file. Note: this is going to work if you send an original message directly to the channel, not as a mock MessageSource origin and if you don't have thread shifting in your flow. So, all the flow is going to be performed in the same thread as your test method. If you have some async hand off in the flow, your MockMessageHandler should do some CountDonwLatch to make the test method to be blocked.

Saying all of these I mean that there are a lot of nuances with testing understanding which comes with the practice and experience. It is probably not possible to do some sample for your which could be useful for other people since there solution might be different and would require other testing approach.

Therefore my advice from here: do your best you know how you could test your own solution.