[Kernel][Testing] Add more thorough testing for DataType.toJson

Feature request

Which Delta project/connector is this regarding?

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• [ ] Spark
• [ ] Standalone
• [ ] Flink
• [X] Kernel
• [ ] Other (fill in here)

Overview

All of our DataType classes provide toJson which serializes the type following the delta protocol schema serialization rules https://github.com/delta-io/delta/blob/master/PROTOCOL.md#schema-serialization-format. We use this method to serialize the physical and logical schemas in ScanStateRow.

toJson is only tested by the 2 round-trip tests in DefaultJsonHandlerSuite. We should add more thorough tests for toJson.