Spark: Reuse core TestTables in Spark tests

[charliec05]

Summary

• remove the duplicated TestTables helper from Spark 3.5, 4.0, and 4.1 tests
• migrate the affected Spark tests to the core org.apache.iceberg.TestTables helper
• load catalog test tables using their metadata location and commit projection-test schema changes through TableOperations

Testing

• ./gradlew :iceberg-spark:iceberg-spark-4.1_2.13:test --tests org.apache.iceberg.spark.source.TestBaseReader --tests org.apache.iceberg.spark.source.TestSparkCatalog --tests org.apache.iceberg.spark.source.TestSparkMetadataColumns --tests org.apache.iceberg.spark.source.TestSparkReadProjection
• ./gradlew -DsparkVersions=4.0 :iceberg-spark:iceberg-spark-4.0_2.13:test --tests org.apache.iceberg.spark.source.TestBaseReader --tests org.apache.iceberg.spark.source.TestSparkCatalog --tests org.apache.iceberg.spark.source.TestSparkMetadataColumns --tests org.apache.iceberg.spark.source.TestSparkReadProjection
• ./gradlew -DsparkVersions=3.5 :iceberg-spark:iceberg-spark-3.5_2.12:test --tests org.apache.iceberg.spark.source.TestBaseReader --tests org.apache.iceberg.spark.source.TestSparkCatalog --tests org.apache.iceberg.spark.source.TestSparkMetadataColumns --tests org.apache.iceberg.spark.source.TestSparkReadProjection
• ./gradlew -DsparkVersions=3.5,4.0,4.1 spotlessCheck

Closes #17508

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AI Disclosure

• Model: GPT-5
• Platform/Tool: Codex
• Human Oversight: unreviewed
• Prompt Summary: Implement Iceberg issue Spark: Remove duplicated TestTables and use core test helper #17508, validate the affected tests across Spark 3.5, 4.0, and 4.1, and prepare a pull request.

[uros-b]

LGTM, thank you @charliec05!