Refs #25746 -- Added a test utility to isolate inlined model registration.

Thanks to Tim for the review.

docs/internals/contributing/writing-code/unit-tests.txt

@@ -303,3 +303,117 @@ purpose.
 
     Support for running tests in parallel and the ``--parallel`` option were
     added.
+
+Tips for writing tests
+----------------------
+
+.. highlight:: python
+
+Isolating model registration
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To avoid polluting the global :attr:`~django.apps.apps` registry and prevent
+unnecessary table creation, models defined in a test method should be bound to
+a temporary ``Apps`` instance::
+
+    from django.apps.registry import Apps
+    from django.db import models
+    from django.test import SimpleTestCase
+
+    class TestModelDefinition(SimpleTestCase):
+        def test_model_definition(self):
+            test_apps = Apps(['app_label'])
+
+            class TestModel(models.Model):
+                class Meta:
+                    apps = test_apps
+            ...
+
+.. function:: django.test.utils.isolate_apps(*app_labels, attr_name=None, kwarg_name=None)
+
+.. versionadded:: 1.10
+
+Since this pattern involves a lot of boilerplate, Django provides the
+:func:`~django.test.utils.isolate_apps` decorator. It's used like this::
+
+    from django.db import models
+    from django.test import SimpleTestCase
+    from django.test.utils import isolate_apps
+
+    class TestModelDefinition(SimpleTestCase):
+        @isolate_apps('app_label')
+        def test_model_definition(self):
+            class TestModel(models.Model):
+                pass
+            ...
+
+.. admonition:: Setting ``app_label``
+
+    Models defined in a test method with no explicit
+    :attr:`~django.db.models.Options.app_label` are automatically assigned the
+    label of the app in which their test class is located.
+
+    In order to make sure the models defined within the context of
+    :func:`~django.test.utils.isolate_apps` instances are correctly
+    installed, you should pass the set of targeted ``app_label`` as arguments:
+
+    .. snippet::
+        :filename: tests/app_label/tests.py
+
+        from django.db import models
+        from django.test import SimpleTestCase
+        from django.test.utils import isolate_apps
+
+        class TestModelDefinition(SimpleTestCase):
+            @isolate_apps('app_label', 'other_app_label')
+            def test_model_definition(self):
+                # This model automatically receives app_label='app_label'
+                class TestModel(models.Model):
+                    pass
+
+                class OtherAppModel(models.Model):
+                    class Meta:
+                        app_label = 'other_app_label'
+                ...
+
+The decorator can also be applied to classes::
+
+    from django.db import models
+    from django.test import SimpleTestCase
+    from django.test.utils import isolate_apps
+
+    @isolate_apps('app_label')
+    class TestModelDefinition(SimpleTestCase):
+        def test_model_definition(self):
+            class TestModel(models.Model):
+                pass
+            ...
+
+The temporary ``Apps`` instance used to isolate model registration can be
+retrieved as an attribute when used as a class decorator by using the
+``attr_name`` parameter::
+
+    from django.db import models
+    from django.test import SimpleTestCase
+    from django.test.utils import isolate_apps
+
+    @isolate_apps('app_label', attr_name='apps')
+    class TestModelDefinition(SimpleTestCase):
+        def test_model_definition(self):
+            class TestModel(models.Model):
+                pass
+            self.assertIs(self.apps.get_model('app_label', 'TestModel'), TestModel)
+
+Or as an argument on the test method when used as a method decorator by using
+the ``kwarg_name`` parameter::
+
+    from django.db import models
+    from django.test import SimpleTestCase
+    from django.test.utils import isolate_apps
+
+    class TestModelDefinition(SimpleTestCase):
+        @isolate_apps('app_label', kwarg_name='apps')
+        def test_model_definition(self, apps):
+            class TestModel(models.Model):
+                pass
+            self.assertIs(apps.get_model('app_label', 'TestModel'), TestModel)