Changed testing strategy used in 6b03179e.

Avoid polluting the app cache as it causes unrelated test failures.

Refs #19688.

tests/modeltests/base/models.py

-from django.db.models.base import ModelBase
+from __future__ import unicode_literals
 
+from django.db import models
+from django.utils import six
 
-class CustomBaseModel(ModelBase):
+
+# The models definitions below used to crash. Generating models dynamically
+# at runtime is a bad idea because it pollutes the app cache. This doesn't
+# integrate well with the test suite but at least it prevents regressions.
+
+
+class CustomBaseModel(models.base.ModelBase):
     pass
+
+
+class MyModel(six.with_metaclass(CustomBaseModel, models.Model)):
+        """Model subclass with a custom base using six.with_metaclass."""
+
+
+if not six.PY3:
+    class MyModel(models.Model):
+        """Model subclass with a custom base using __metaclass__."""
+        __metaclass__ = CustomBaseModel

tests/modeltests/base/tests.py

-from __future__ import unicode_literals
-
-from django.db import models
-from django.test.testcases import SimpleTestCase
-from django.utils import six
-from django.utils.unittest import skipIf
-
-from .models import CustomBaseModel
-
-
-class CustomBaseTest(SimpleTestCase):
-
-    @skipIf(six.PY3, 'test metaclass definition under Python 2')
-    def test_py2_custom_base(self):
-        """
-        Make sure models.Model can be subclassed with a valid custom base
-        using __metaclass__
-        """
-        try:
-            class MyModel(models.Model):
-                __metaclass__ = CustomBaseModel
-        except Exception:
-            self.fail("models.Model couldn't be subclassed with a valid "
-                      "custom base using __metaclass__.")
-
-    def test_six_custom_base(self):
-        """
-        Make sure models.Model can be subclassed with a valid custom base
-        using `six.with_metaclass`.
-        """
-        try:
-            class MyModel(six.with_metaclass(CustomBaseModel, models.Model)):
-                pass
-        except Exception:
-            self.fail("models.Model couldn't be subclassed with a valid "
-                      "custom base using `six.with_metaclass`.")