Unify testing utilities

[sebastiantia]

Please describe why this is necessary.

We have repeated definitions of testing utilities such as into_record_batch defined in both the top-level kernel and test-utils crate, along with other examples such as repeated definitions of string_array_to_engine_data that should be merged.

This duplication increases maintenance overhead and can lead to inconsistencies between test implementations.

Describe the functionality you are proposing.

• Consolidate duplicated testing utilities into a single, well-defined location (likely test-utils).
• Ensure that all test code references the unified utilities rather than maintaining separate versions.
• Where applicable, refactor existing tests to use the centralized implementations to improve maintainability and readability.

Additional context

No response