[tests] decorate StableDiffusion21PipelineSingleFileSlowTests with slow. (#7941)

decorate StableDiffusion21PipelineSingleFileSlowTests with slow.

Author: sayakpaul

tests/single_file/test_stable_diffusion_single_file.py

@@ -82,6 +82,7 @@ def test_single_file_legacy_scaling_factor(self):
         assert pipe.vae.config.scaling_factor == new_scaling_factor
 
 
+@slow
 class StableDiffusion21PipelineSingleFileSlowTests(unittest.TestCase, SDSingleFileTesterMixin):
     pipeline_class = StableDiffusionPipeline
     ckpt_path = "https://huggingface.co/stabilityai/stable-diffusion-2-1/blob/main/v2-1_768-ema-pruned.safetensors"