SNOW-1959569: Fix test_datatype_suite.py::test_structured_dtypes_select

[sfc-gh-jrose]

1. Which Jira issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-1959569

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
     • If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am adding new credentials
   • I am adding a new dependency
   • If this is a new feature/behavior, I'm adding the Local Testing parity changes.
   • I acknowledge that I have ensured my changes to be thread-safe. Follow the link for more information: Thread-safe Developer Guidelines

3. Please describe how your code solves the related issue.

   This PR fixes two tests:

   • test_structured_dtypes_select: The _create_test_dataframe that sets up the dataframe for this test was using structured types regardless of if the server side had them enabled. This created a difference in expected schema between what the describe reducer expected and what the server reported. I've updated the test to only use structured types when enabled server side and when it is not the expected VariantTypes are returned.
   • test_structured_dtypes_iceberg: I'm not sure why the ddl for this test changed, but it appears to be more accurate on the server side now so I suspect something changed there.

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