code: def test_get_variable_secret_string(self):
      param = {
          "Name": "/airflow/variables/hello",
          "Type": "SecureString",
          "Value": "world",
      }
      ssm_backend = SystemsManagerParameterStoreBackend()
      ssm_backend.client.put_parameter(**param)
      returned_uri = ssm_backend.get_variable("hello")
      assert "world" == returned_uri
  
imports: from __future__ import annotations
  import json
  from unittest import mock
  import pytest
  from moto import mock_ssm
  from airflow.configuration import initialize_secrets_backends
  from airflow.providers.amazon.aws.secrets.systems_manager import SystemsManagerParameterStoreBackend
  from tests.test_utils.config import conf_vars
label_desc: Use image node_id 6 for calling the TestSsmSecrets obj's underlying member method code with example usage: obj.test_get_variable_secret_string() without return types