[FLINK-33371] Make TestValues sinks return results as Rows

[dawidwys]

What is the purpose of the change

If we want to use the predicates from #23584 in restore tests we need to make testing sinks return Rows instead of Strings

Verifying this change

All existing tests still work.

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:

• c73b8f0 Azure: SUCCESS

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[twalthr]

I found one issue. Otherwise should be good if tests are green.

[twalthr]

LGTM

[twalthr]

One last comment.