Fixed #25746 -- Isolated inlined test models registration.

Thanks to Tim for the review.

tests/str/tests.py

@@ -4,9 +4,9 @@ from __future__ import unicode_literals
 import datetime
 from unittest import skipIf
 
-from django.apps.registry import Apps
 from django.db import models
 from django.test import TestCase
+from django.test.utils import isolate_apps
 from django.utils import six
 
 from .models import Article, InternationalArticle
@@ -36,16 +36,14 @@ class SimpleTests(TestCase):
             # python_2_unicode_compatible decorator is used.
             self.assertEqual(str(a), b'Girl wins \xe2\x82\xac12.500 in lottery')
 
+    @isolate_apps('str')
     def test_defaults(self):
         """
         The default implementation of __str__ and __repr__ should return
         instances of str.
         """
-        test_apps = Apps(['str'])
-
         class Default(models.Model):
-            class Meta:
-                apps = test_apps
+            pass
 
         obj = Default()
         # Explicit call to __str__/__repr__ to make sure str()/repr() don't