code: def test_get_variable(self, mock_client):
      mock_client.get_secret.return_value = mock.Mock(value="world")
      backend = AzureKeyVaultBackend()
      returned_uri = backend.get_variable("hello")
      mock_client.get_secret.assert_called_with(
          name="airflow-variables-hello"
      )
      assert "world" == returned_uri
  
file_path: airflow/tests/providers/microsoft/azure/secrets/test_key_vault.py
imports: from __future__ import annotations
  from unittest import mock
  import pytest
  from azure.core.exceptions import ResourceNotFoundError
  from airflow.exceptions import AirflowProviderDeprecationWarning
  from airflow.providers.microsoft.azure.secrets.key_vault import AzureKeyVaultBackend
label_desc: Use image node_id 4 for calling the TestAzureKeyVaultBackend obj's underlying member method code with example usage: obj.test_get_variable(mock_client) without return types