Add tests for casting between timestamp types

[andygrove]

What is the problem the feature request solves?

We currently delegate to DataFusion when casting between timestamps (as discovered in #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

[parthchandra]

In CometExpressionSuite we have -
test("cast timestamp and timestamp_ntz") (long -> timestamp/timestamp_ntz)
test("cast timestamp and timestamp_ntz to string")
test("cast timestamp and timestamp_ntz to long, date")

Do we want to add more values in these tests or do we want some more combinations tested (which ones)?