Fixed #25746 -- Isolated inlined test models registration.

Thanks to Tim for the review.

tests/managers_regress/tests.py

@@ -1,9 +1,9 @@
 from __future__ import unicode_literals
 
-from django.apps import apps
 from django.db import models
 from django.template import Context, Template
 from django.test import TestCase, override_settings
+from django.test.utils import isolate_apps
 from django.utils.encoding import force_text
 
 from .models import (
@@ -91,82 +91,58 @@ class ManagersRegressionTests(TestCase):
             AbstractBase1.objects.all()
 
     @override_settings(TEST_SWAPPABLE_MODEL='managers_regress.Parent')
+    @isolate_apps('managers_regress')
     def test_swappable_manager(self):
-        # The models need to be removed after the test in order to prevent bad
-        # interactions with the flush operation in other tests.
-        _old_models = apps.app_configs['managers_regress'].models.copy()
+        class SwappableModel(models.Model):
+            class Meta:
+                swappable = 'TEST_SWAPPABLE_MODEL'
 
-        try:
-            class SwappableModel(models.Model):
-                class Meta:
-                    swappable = 'TEST_SWAPPABLE_MODEL'
-
-            # Accessing the manager on a swappable model should
-            # raise an attribute error with a helpful message
-            msg = (
-                "Manager isn't available; 'managers_regress.SwappableModel' "
-                "has been swapped for 'managers_regress.Parent'"
-            )
-            with self.assertRaisesMessage(AttributeError, msg):
-                SwappableModel.objects.all()
-        finally:
-            apps.app_configs['managers_regress'].models = _old_models
-            apps.all_models['managers_regress'] = _old_models
-            apps.clear_cache()
+        # Accessing the manager on a swappable model should
+        # raise an attribute error with a helpful message
+        msg = (
+            "Manager isn't available; 'managers_regress.SwappableModel' "
+            "has been swapped for 'managers_regress.Parent'"
+        )
+        with self.assertRaisesMessage(AttributeError, msg):
+            SwappableModel.objects.all()
 
     @override_settings(TEST_SWAPPABLE_MODEL='managers_regress.Parent')
+    @isolate_apps('managers_regress')
     def test_custom_swappable_manager(self):
-        # The models need to be removed after the test in order to prevent bad
-        # interactions with the flush operation in other tests.
-        _old_models = apps.app_configs['managers_regress'].models.copy()
+        class SwappableModel(models.Model):
+            stuff = models.Manager()
 
-        try:
-            class SwappableModel(models.Model):
-                stuff = models.Manager()
+            class Meta:
+                swappable = 'TEST_SWAPPABLE_MODEL'
 
-                class Meta:
-                    swappable = 'TEST_SWAPPABLE_MODEL'
-
-            # Accessing the manager on a swappable model with an
-            # explicit manager should raise an attribute error with a
-            # helpful message
-            msg = (
-                "Manager isn't available; 'managers_regress.SwappableModel' "
-                "has been swapped for 'managers_regress.Parent'"
-            )
-            with self.assertRaisesMessage(AttributeError, msg):
-                SwappableModel.stuff.all()
-        finally:
-            apps.app_configs['managers_regress'].models = _old_models
-            apps.all_models['managers_regress'] = _old_models
-            apps.clear_cache()
+        # Accessing the manager on a swappable model with an
+        # explicit manager should raise an attribute error with a
+        # helpful message
+        msg = (
+            "Manager isn't available; 'managers_regress.SwappableModel' "
+            "has been swapped for 'managers_regress.Parent'"
+        )
+        with self.assertRaisesMessage(AttributeError, msg):
+            SwappableModel.stuff.all()
 
     @override_settings(TEST_SWAPPABLE_MODEL='managers_regress.Parent')
+    @isolate_apps('managers_regress')
     def test_explicit_swappable_manager(self):
-        # The models need to be removed after the test in order to prevent bad
-        # interactions with the flush operation in other tests.
-        _old_models = apps.app_configs['managers_regress'].models.copy()
+        class SwappableModel(models.Model):
+            objects = models.Manager()
 
-        try:
-            class SwappableModel(models.Model):
-                objects = models.Manager()
+            class Meta:
+                swappable = 'TEST_SWAPPABLE_MODEL'
 
-                class Meta:
-                    swappable = 'TEST_SWAPPABLE_MODEL'
-
-            # Accessing the manager on a swappable model with an
-            # explicit manager should raise an attribute error with a
-            # helpful message
-            msg = (
-                "Manager isn't available; 'managers_regress.SwappableModel' "
-                "has been swapped for 'managers_regress.Parent'"
-            )
-            with self.assertRaisesMessage(AttributeError, msg):
-                SwappableModel.objects.all()
-        finally:
-            apps.app_configs['managers_regress'].models = _old_models
-            apps.all_models['managers_regress'] = _old_models
-            apps.clear_cache()
+        # Accessing the manager on a swappable model with an
+        # explicit manager should raise an attribute error with a
+        # helpful message
+        msg = (
+            "Manager isn't available; 'managers_regress.SwappableModel' "
+            "has been swapped for 'managers_regress.Parent'"
+        )
+        with self.assertRaisesMessage(AttributeError, msg):
+            SwappableModel.objects.all()
 
     def test_regress_3871(self):
         related = RelatedModel.objects.create()