Fix license header for range plugin tests

Fixes: da62c7b1bd5622e127f9e664659299972274e048

Signed-off-by: Anatole Denis <anatole@unverle.fr>

plugins/range/plugin_test.go

@@ -1,3 +1,7 @@
+// Copyright 2018-present the CoreDHCP Authors. All rights reserved
+// This source code is licensed under the MIT license found in the
+// LICENSE file in the root directory of this source tree.
+
 package rangeplugin
 
 import (