[FLINK-11185] Fix StreamSourceOperatorWatermarksTest instability.

[kl0u]

The change removes the mainThread.interrupt() call from the StreamSourceOperatorWatermarksTest.testNoMaxWatermarkOnAsyncCancel() test.

The cause of the instability seems to be that due to a not-so-rare timing, the thread that calls the interrupt() on the main thread, runs still after its original test finishes and call interrupt() during execution of the next test. This causes the normal execution (or sleep() in this case) to be interrupted.

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[aljoscha]

This looks good! Can you add the description from this PR to the commit itself?

[kl0u]

I will. Thanks for the review @aljoscha

[kl0u]

Can you also accept the PR (for the record)