Add test for `as_iterable_dataset()` method in DatasetBuilder
This PR adds a test for the new `as_iterable_dataset()` method introduced in PR #7628.

The test:
- Loads a builder using `load_dataset_builder("c4", "en")`
- Runs `download_and_prepare()`
- Streams examples using `builder.as_iterable_dataset(split="train[:100]")`
- Verifies streamed examples contain the "text" field

This ensures that the builder correctly streams data from cached Arrow files.
