Fixed #25160 -- Moved unsaved model instance data loss check to Model.save()

This mostly reverts 5643a3b51b and
81e1a35c36.

Thanks Carl Meyer for review.

tests/admin_utils/tests.py

@@ -12,7 +12,7 @@ from django.contrib.admin.utils import (
     label_for_field, lookup_field, quote,
 )
 from django.db import DEFAULT_DB_ALIAS, models
-from django.test import TestCase, override_settings
+from django.test import SimpleTestCase, TestCase, override_settings
 from django.utils import six
 from django.utils.formats import localize
 from django.utils.safestring import mark_safe
@@ -94,7 +94,7 @@ class NestedObjectsTests(TestCase):
         n.collect([Vehicle.objects.first()])
 
 
-class UtilsTests(TestCase):
+class UtilsTests(SimpleTestCase):
 
     empty_value = '-empty-'
 
@@ -115,7 +115,7 @@ class UtilsTests(TestCase):
 
         simple_function = lambda obj: SIMPLE_FUNCTION
 
-        site_obj = Site.objects.create(domain=SITE_NAME)
+        site_obj = Site(domain=SITE_NAME)
         article = Article(
             site=site_obj,
             title=TITLE_TEXT,