[FLINK-39326][runtime] Fix DispatcherApplicationTest.testJobResultNot…

[eemario]

…MarkedCleanUntilApplicationTerminates

What is the purpose of the change

This pull request addresses a flaky test: DispatcherApplicationTest.testJobResultNotMarkedCleanUntilApplicationTerminates.
The test occasionally fails because the assertion that a dirty job result exists may run before the job result is actually created.
To resolve this, the test has been updated to wait for the job result to be created before performing the subsequent validation.

Brief change log

• Improve the test validation logic

Verifying this change

Please make sure both new and modified tests in this PR follow the conventions for tests defined in our code quality guide.

This change is already covered by existing tests, such as DispatcherApplicationTest.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[flinkbot]

CI report:

• aaece41 Azure: SUCCESS

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