[tests] Use load_model for Config instead of direct import

Use the swappable-model pattern (load_model) for Config at module
level, consistent with Device/DeviceGroup/Template in the same file.

Author: mn-ram

openwisp_controller/config/tests/test_device_group.py

@@ -11,6 +11,7 @@
 from ..signals import group_templates_changed
 from .utils import CreateDeviceGroupMixin, CreateDeviceMixin, CreateTemplateMixin
 
+Config = load_model("config", "Config")
 Device = load_model("config", "Device")
 DeviceGroup = load_model("config", "DeviceGroup")
 Template = load_model("config", "Template")
@@ -96,8 +97,6 @@ def test_manage_devices_group_templates_no_early_termination(self):
             organization=org,
             mac_address="00:00:00:00:00:02",
         )
-        from openwisp_controller.config.tests.utils import Config
-
         Config.objects.create(
             device=device2,
             backend="netjsonconfig.OpenWrt",