[FLINK-39158][runtime/test] Minor polish for RestAPIStabilityTestUtils to reduce redundant lines

[RocMarshal]

What is the purpose of the change

Minor polish for RestAPIStabilityTestUtils to reduce redundant lines

Brief change log

Minor polish for RestAPIStabilityTestUtils to reduce redundant lines

Verifying this change

This change is already covered by existing tests

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

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

[flinkbot]

CI report:

• c856946 Azure: FAILURE

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[RocMarshal]

Thanks @featzhang for review.
Merging...