Add 'This file is part of Kerolox.' to AST test license header

crates/frontend/tests/ast.rs

@@ -1,6 +1,8 @@
 // Copyright (C) 2025-2026 Marceline Cramer
 // SPDX-License-Identifier: AGPL-3.0-or-later
 //
+// This file is part of Kerolox.
+//
 // Kerolox is free software: you can redistribute it and/or modify it under
 // the terms of the GNU Affero General Public License as published by the Free
 // Software Foundation, either version 3 of the License, or (at your option) any