Fix incorrect argument name in test

This test wasn’t checking the redirect because the argument name was
missing the leading underscore.

tests/app/main/views/test_send.py

@@ -565,7 +565,14 @@ def test_upload_valid_csv_redirects_to_check_page(
         'main.send_messages', service_id=SERVICE_ONE_ID, template_id=fake_uuid,
         _data={'file': (BytesIO(''.encode('utf-8')), 'valid.csv')},
         _expected_status=302,
-        expected_redirect='foo'
+        _expected_redirect=url_for(
+            'main.check_messages',
+            service_id=SERVICE_ONE_ID,
+            template_id=fake_uuid,
+            upload_id=fake_uuid,
+            original_file_name='valid.csv',
+            _external=True,
+        ),
     )