Added a missing test for createsuperuser management command.

tests/auth_tests/test_management.py

@@ -701,6 +701,34 @@ class CreatesuperuserManagementCommandTestCase(TestCase):
                 stdout=new_io,
             )
 
+    def test_existing_username_provided_via_option_and_interactive(self):
+        """call_command() gets username='janet' and interactive=True."""
+        new_io = StringIO()
+        entered_passwords = ['password', 'password']
+        User.objects.create(username='janet')
+
+        def return_passwords():
+            return entered_passwords.pop(0)
+
+        @mock_inputs({
+            'password': return_passwords,
+            'username': 'janet1',
+            'email': 'test@test.com'
+        })
+        def test(self):
+            call_command(
+                'createsuperuser',
+                username='janet',
+                interactive=True,
+                stdin=MockTTY(),
+                stdout=new_io,
+                stderr=new_io,
+            )
+            msg = 'Error: That username is already taken.\nSuperuser created successfully.'
+            self.assertEqual(new_io.getvalue().strip(), msg)
+
+        test(self)
+
     def test_validation_mismatched_passwords(self):
         """
         Creation should fail if the user enters mismatched passwords.