Fixed #25746 -- Isolated inlined test models registration.

Thanks to Tim for the review.

tests/invalid_models_tests/test_custom_fields.py

@@ -1,9 +1,10 @@
 from django.db import models
-
-from .base import IsolatedModelsTestCase
+from django.test import SimpleTestCase
+from django.test.utils import isolate_apps
 
 
-class CustomFieldTest(IsolatedModelsTestCase):
+@isolate_apps('invalid_models_tests')
+class CustomFieldTest(SimpleTestCase):
 
     def test_none_column(self):
         class NoColumnField(models.AutoField):