12794 change User ref to get_user_model (#12905)

* 12794 change User ref to get_user_model

* 12794 call get_user_model once in tests

* 12794 call get_user_model once in tests

* 12794 use settings.AUTH_USER_MODEL for FK reference

netbox/utilities/testing/api.py

@@ -2,7 +2,7 @@ import inspect
 import json
 
 from django.conf import settings
-from django.contrib.auth.models import User
+from django.contrib.auth import get_user_model
 from django.contrib.contenttypes.models import ContentType
 from django.urls import reverse
 from django.test import override_settings
@@ -26,6 +26,9 @@ __all__ = (
 )
 
 
+User = get_user_model()
+
+
 #
 # REST/GraphQL API Tests
 #