tests: add _setup_image_processor to Donut with size=64x64 to reduce memory
Default DonutImageProcessor uses size=2560×1920 (document scan resolution), producing
~59 MB pixel_values per image as float32. No test assertions check spatial dimensions,
so reducing to 64×64 cuts test_image_processor_defaults from ~106 MB to near zero.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>