[hotfix][tests] Add computeDigest to DummyStructuredType

[snuyanzin]

What is the purpose of the change

This is another step towards Calcite 1.42.0
This is required after https://issues.apache.org/jira/browse/CALCITE-7393

Brief change log

tests

Verifying this change

existing tests

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 )

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CI report:

• 30c7ff0 Azure: SUCCESS

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