Fixed #25746 -- Isolated inlined test models registration.

Thanks to Tim for the review.

tests/sites_framework/tests.py

@@ -1,14 +1,12 @@
-from django.apps import apps
 from django.conf import settings
 from django.contrib.sites.managers import CurrentSiteManager
 from django.contrib.sites.models import Site
 from django.core import checks
 from django.db import models
-from django.test import TestCase
+from django.test import SimpleTestCase, TestCase
+from django.test.utils import isolate_apps
 
-from .models import (
-    AbstractArticle, CustomArticle, ExclusiveArticle, SyndicatedArticle,
-)
+from .models import CustomArticle, ExclusiveArticle, SyndicatedArticle
 
 
 class SitesFrameworkTestCase(TestCase):
@@ -16,13 +14,6 @@ class SitesFrameworkTestCase(TestCase):
         Site.objects.get_or_create(id=settings.SITE_ID, domain="example.com", name="example.com")
         Site.objects.create(id=settings.SITE_ID + 1, domain="example2.com", name="example2.com")
 
-        self._old_models = apps.app_configs['sites_framework'].models.copy()
-
-    def tearDown(self):
-        apps.app_configs['sites_framework'].models = self._old_models
-        apps.all_models['sites_framework'] = self._old_models
-        apps.clear_cache()
-
     def test_site_fk(self):
         article = ExclusiveArticle.objects.create(title="Breaking News!", site_id=settings.SITE_ID)
         self.assertEqual(ExclusiveArticle.on_site.all().get(), article)
@@ -42,12 +33,12 @@ class SitesFrameworkTestCase(TestCase):
         )
         self.assertEqual(CustomArticle.on_site.all().get(), article)
 
+
+@isolate_apps('sites_framework')
+class CurrentSiteManagerChecksTests(SimpleTestCase):
+
     def test_invalid_name(self):
-
-        class InvalidArticle(AbstractArticle):
-            site = models.ForeignKey(Site, models.CASCADE)
-
-            objects = models.Manager()
+        class InvalidArticle(models.Model):
             on_site = CurrentSiteManager("places_this_article_should_appear")
 
         errors = InvalidArticle.check()
@@ -64,8 +55,9 @@ class SitesFrameworkTestCase(TestCase):
 
     def test_invalid_field_type(self):
 
-        class ConfusedArticle(AbstractArticle):
+        class ConfusedArticle(models.Model):
             site = models.IntegerField()
+            on_site = CurrentSiteManager()
 
         errors = ConfusedArticle.check()
         expected = [