Update and tests for %RegExpPrototype% methods

A recent web-compatability change to ECMA262 modified the semantics of
the accessor methods on the %RegExpPrototype% intrinsic--the "get"
accessors now include steps dedicated to the case where the "this" value
is the %RegExpPrototype% object itself.

Remove the tests that have been invalidated by this change, introduce
tests asserting the new behavior, and extend coverage for other possible
"this" values.

test/built-ins/RegExp/prototype/unicode/this-invald-obj.js → test/built-ins/RegExp/prototype/unicode/this-val-invalid-obj.js

@@ -17,4 +17,12 @@ var unicode = Object.getOwnPropertyDescriptor(RegExp.prototype, 'unicode').get;
 
 assert.throws(TypeError, function() {
   unicode.call({});
-});
+}, 'ordinary object');
+
+assert.throws(TypeError, function() {
+  unicode.call([]);
+}, 'array exotic object');
+
+assert.throws(TypeError, function() {
+  unicode.call(arguments);
+}, 'arguments object');

test/built-ins/RegExp/prototype/unicode/this-val-regexp-prototype.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-get-regexp.prototype.unicode
+description: >
+  Return value of `undefined` when the "this" value is the RegExp prototype
+  object
+info: |
+  1. Let R be the this value.
+  2. If Type(R) is not Object, throw a TypeError exception.
+  3. If R does not have an [[OriginalFlags]] internal slot, then
+     a. If SameValue(R, %RegExpPrototype%) is true, return undefined.
+---*/
+
+var get = Object.getOwnPropertyDescriptor(RegExp.prototype, 'unicode').get;
+
+assert.sameValue(get.call(RegExp.prototype), undefined);