go/ast/internal/tests: add missing copyright header

I forgot to add it in CL 616340

Change-Id: I6c906fa79f2e53ce8bf506d836472b037cb46f85

src/go/ast/internal/tests/sortimports_test.go

@@ -1,3 +1,7 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 // Tests is a helper package to avoid cyclic dependency between go/ast and go/parser.
 package tests