Add tests for casting between timestamp types

What is the problem the feature request solves?

We currently delegate to DataFusion when casting between timestamps (as discovered in https://github.com/apache/datafusion-comet/pull/461) but I am not convinced that we have comprehensive tests to ensure that this is compatible with Spark.

This issue is for adding specific tests for casting between timestamp types.

Describe the potential solution

No response

Additional context

No response