add license header to file created in prev commit

tests/test_splitk_reference.py

@@ -1,3 +1,8 @@
+# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+#
+# This source code is licensed under the BSD license found in the
+# LICENSE file in the root directory of this source tree.
+
 from typing import Optional, Tuple
 
 import pytest