Add `global` tests. (#765)

Closes #605, relates to #567 / https://github.com/tc39/proposal-global/issues/12 / https://github.com/bocoup/test262/commit/9c45e2ac684bae64614d8eb55789cae97323a7e7

test/built-ins/global/global-object.js

+// Copyright (C) 2016 Jordan Harband. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-other-properties-of-the-global-object-global
+description: `global` should be the global object
+author: Jordan Harband
+includes: [propertyHelper.js]
+---*/
+
+assert.sameValue(this, global);
+assert.sameValue(global.global, global);
+
+assert.sameValue(Array, global.Array);
+assert.sameValue(Boolean, global.Boolean);
+assert.sameValue(Date, global.Date);
+assert.sameValue(Error, global.Error);
+assert.sameValue(Function, global.Function);
+assert.sameValue(JSON, global.JSON);
+assert.sameValue(Math, global.Math);
+assert.sameValue(Number, global.Number);
+assert.sameValue(RegExp, global.RegExp);
+assert.sameValue(String, global.String);
+
+globalVariable = {};
+assert.sameValue(globalVariable, global.globalVariable);

test/built-ins/global/property-descriptor.js

+// Copyright (C) 2016 Jordan Harband. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-other-properties-of-the-global-object-global
+description: `global` should be writable, non-enumerable, and configurable
+author: Jordan Harband
+includes: [propertyHelper.js]
+---*/
+
+verifyNotEnumerable(this, 'global');
+verifyWritable(this, 'global');
+verifyConfigurable(this, 'global');