Fixed #25746 -- Isolated inlined test models registration.

Thanks to Tim for the review.

tests/invalid_models_tests/test_deprecated_fields.py

@@ -1,10 +1,11 @@
 from django.core import checks
 from django.db import models
-
-from .base import IsolatedModelsTestCase
+from django.test import SimpleTestCase
+from django.test.utils import isolate_apps
 
 
-class DeprecatedFieldsTests(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class DeprecatedFieldsTests(SimpleTestCase):
     def test_IPAddressField_deprecated(self):
         class IPAddressModel(models.Model):
             ip = models.IPAddressField()