Flink: fix flaky test that might fail due to classloader check

[stevenzwu]

No description provided.

[stevenzwu]

see an example of failed test here: https://github.com/apache/iceberg/actions/runs/7091452262/job/19300609355?pr=9212

    exception caught while trying to get the future result: java.util.concurrent.ExecutionException: java.lang.IllegalStateException: Trying to access closed classloader. Please check if you store classloaders directly or indirectly in static fields. If the stacktrace suggests that the leak occurs in a third party library and cannot be fixed immediately, you can disable this check with the configuration 'classloader.check-leaked-classloader'.
    	at java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:357)
    	at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1928)
    	at org.assertj.core.internal.Futures.assertSucceededWithin(Futures.java:109)
    	at org.assertj.core.api.AbstractCompletableFutureAssert.internalSucceedsWithin(AbstractCompletableFutureAssert.java:400)
    	at org.assertj.core.api.AbstractCompletableFutureAssert.succeedsWithin(AbstractCompletableFutureAssert.java:396)
    	at org.apache.iceberg.flink.source.TestIcebergSourceWithWatermarkExtractor.waitForRecords(TestIcebergSourceWithWatermarkExtractor.java:416)
    	at org.apache.iceberg.flink.source.TestIcebergSourceWithWatermarkExtractor.testThrottling(TestIcebergSourceWithWatermarkExtractor.java:307)

[pvary]

Thanks for catching and fixing this @stevenzwu !

[ajantha-bhat]

I think this is not fully fixed.

With the latest code, it has failed again
https://github.com/apache/iceberg/actions/runs/7208855885/job/19638664888?pr=9296

cc: @nastra, @stevenzwu, @pvary

[ajantha-bhat]

@stevenzwu: Looks like the code is already there in 1.16, but still it is failed. Please take a look.

[stevenzwu]

@pvary can you follow up? if this can't be fixed quickly, let's ignore this test for now

[pvary]

I was originally not able to run the tests multiple times using IntelliJ, but found some issues, and fixed them.
Now I have a test which I was able to run 1000 times from IntelliJ without errors.
Let me prepare the PR