Fix native engine vector format test

[VijayanB]

Description

Previosuly we were not creating hnsw file on segment flush for faiss engine. After successfully integrating hnsw file creation, we forgot to update unit test. In this PR, we will confirm that required files are being created based on field type.

Related Issues

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [x] Commits are signed per the DCO using --signoff.
• [ ] Public documentation issue/PR created.

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[navneet1v]

Thanks @VijayanB for making the test tighter. Somehow when integration happened with NativeIndexWriter the update of this test was missed. Good catch. Approving the code. Assuming tests will pass.