Fixed #28751 -- Corrected the error message for inactive users in AdminAuthenticationForm.

Thanks SeungWon Kang for the report and Tim Graham for the review.

tests/admin_views/test_forms.py

@@ -0,0 +1,17 @@
+from django.contrib.admin.forms import AdminAuthenticationForm
+from django.contrib.auth.models import User
+from django.test import TestCase
+
+
+class AdminAuthenticationFormTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        User.objects.create_user(username='inactive', password='password', is_active=False)
+
+    def test_inactive_user(self):
+        data = {
+            'username': 'inactive',
+            'password': 'password',
+        }
+        form = AdminAuthenticationForm(None, data)
+        self.assertEqual(form.non_field_errors(), ['This account is inactive.'])