[1.6.x] Imported custom user classes in tests depending on it

Without those imports, affected test files cannot be run
independently. Refs #21164.

Backport of ef22d512b5 from master.

django/contrib/auth/tests/test_models.py

@@ -4,6 +4,7 @@ from django.conf import settings
 from django.contrib.auth import get_user_model
 from django.contrib.auth.models import (Group, User, SiteProfileNotAvailable,
     UserManager)
+from django.contrib.auth.tests.custom_user import IsActiveTestUser1
 from django.contrib.auth.tests.utils import skipIfCustomUser
 from django.db.models.signals import post_save
 from django.test import TestCase

django/contrib/auth/tests/test_views.py

@@ -21,6 +21,7 @@ from django.contrib.sessions.middleware import SessionMiddleware
 from django.contrib.auth import SESSION_KEY, REDIRECT_FIELD_NAME
 from django.contrib.auth.forms import (AuthenticationForm, PasswordChangeForm,
                 SetPasswordForm)
+from django.contrib.auth.tests.custom_user import CustomUser
 from django.contrib.auth.tests.utils import skipIfCustomUser
 from django.contrib.auth.views import login as login_view