[CI] Add pytest markers to current tests and update the doc. (#577)

Signed-off-by: Alicia <115451386+congw729@users.noreply.github.com>
Co-authored-by: Hongsheng Liu <liuhongsheng4@huawei.com>

tests/entrypoints/openai_api/test_image_server.py

@@ -23,6 +23,8 @@ from vllm_omni.entrypoints.openai.image_api_utils import (
 )
 from vllm_omni.inputs.data import OmniDiffusionSamplingParams
 
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
 # Unit Tests