[FLINK-23406][tests] Harden ClusterUncaughtExceptionHandlerITCase.testExitDueToUncaughtException

[tillrohrmann]

This commit hardens the ClusterUncaughtExceptionHandlerITCase.testExitDueToUncaughtException by not blocking
the ClusterEntrypoint's main thread and therefore releasing the ClusterEntrypoint.lock.

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[zentol]

+1

[tillrohrmann]

Thanks for the review @zentol. Merging this PR now.