Fix posix path issue in testing utils (#6849)

update

src/diffusers/utils/testing_utils.py

@@ -522,7 +522,7 @@ def load_hf_numpy(path) -> np.ndarray:
     base_url = "https://huggingface.co/datasets/fusing/diffusers-testing/resolve/main"
 
     if not path.startswith("http://") and not path.startswith("https://"):
-        path = Path(base_url, urllib.parse.quote(path)).as_posix()
+        path = os.path.join(base_url, urllib.parse.quote(path))
 
     return load_numpy(path)