message: nydus-test: ignore missing backend_proxy_url
  backend_proxy_url is not mandatory.
diff: @@ -55,10 +55,13 @@ class NydusAnchor:
  registry_conf = kwargs.pop("registry")
  self.registry_url = registry_conf["registry_url"]
  - self.backend_proxy_url = registry_conf["backend_proxy_url"]
  self.registry_auth = registry_conf["registry_auth"]
  self.registry_namespace = registry_conf["registry_namespace"]
  + try:
  + self.backend_proxy_url = registry_conf["backend_proxy_url"]
  self.backend_proxy_blobs_dir = registry_conf["backend_proxy_blobs_dir"]
  + except KeyError:
  + pass
  os.makedirs(self.backend_proxy_blobs_dir, exist_ok=True)
  