refactor(test): reorganize file connector CI tests

[wcy-fdu]

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What's changed and what's your intention?

The previous tests for file source and file sink were quite complicated. This PR reorganized them and put all the functional and correctness tests together.

• file_source.py: test streaming source on json and csv encode, batch read from file source and handling incremental files.
• file_sink.pr: test sink into file system in both parquet and json encode, and test the batching strategy.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added test labels as necessary. See details.
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
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• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

Documentation

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Release note

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