Add tests for `await` as an identifier

test/language/reserved-words/await-module.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+id: sec-reserved-words
+es6id: 11.6.2
+description: The `await` token is not permitted as an identifier in module code
+flags: [module]
+negative: SyntaxError
+---*/
+
+var await;

test/language/reserved-words/await-script.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+id: sec-reserved-words
+es6id: 11.6.2
+description: The `await` token is permitted as an identifier in script code
+---*/
+
+var await;