[Misc] Refactor VLM common generation tests to support audio inputs and mix-modality tests

[Isotr0py]

• After [CI/Build] Reorganize models tests #17459, we should add mixed modality tests for omni models, since we no longer group tests by modality.
• This PR refactors the generation/test_common.py tests to support audio inputs for mixed modality tests.
• This PR also adds Image-Audio mixed-modality tests for Qwen2.5-omni

[github-actions]

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[DarkLight1337]

Let's wait for transformers 4.52.0 so this test is actually being run in CI