Unskip FP8 Z-Image quality case

Signed-off-by: WeiQing Chen <david6666666@users.noreply.github.com>

tests/diffusion/quantization/artifacts/fp8_z_image_quality/README.md

@@ -0,0 +1,29 @@
+# FP8 Z-Image Quality Artifact
+
+Generated by:
+
+```bash
+CUDA_VISIBLE_DEVICES=0 \
+VLLM_OMNI_QUALITY_OUTPUT_DIR=$PWD/tests/diffusion/quantization/artifacts/fp8_z_image_quality \
+.venv/bin/python -m pytest \
+  tests/diffusion/quantization/test_quantization_quality.py::test_quantization_quality[fp8_z_image] \
+  -s -v -m ""
+```
+
+Result: failed with the current `max_lpips=0.15` threshold.
+
+| Metric | Value |
+|--------|-------|
+| LPIPS | 0.250578 |
+| PSNR | 17.042153 dB |
+| MAE | 0.076899 |
+
+## Comparison
+
+![BF16 baseline vs FP8 quantized](fp8_z_image_comparison.png)
+
+## Individual Outputs
+
+![BF16 baseline](fp8_z_image_baseline.png)
+
+![FP8 quantized](fp8_z_image_quantized.png)