Add omitted license information

As the author of these files, I can verify that they were contributed in
2015 on behalf of the V8 project.

test/built-ins/Promise/race/resolve-poisoned-then.js

+// Copyright (C) 2015 the V8 project authors. All rights reserved.
 // This code is governed by the BSD license found in the LICENSE file.
 /*---
 description: Resolving with an object with a "poisoned" `then` property

test/built-ins/Promise/race/resolve-self.js

+// Copyright (C) 2015 the V8 project authors. All rights reserved.
 // This code is governed by the BSD license found in the LICENSE file.
 /*---
 description: Resolving with a reference to the promise itself