Assert absense of internal slot on RegExpPrototype

The previous commit removed two invalid tests designed to ensure that
the RegExpPrototype object is not itself a RegExp object. Introduce a
new test to assert this detail in a way that accounts for the
possibility of host extensions.

test/built-ins/RegExp/prototype/no-regexp-matcher.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-properties-of-the-regexp-prototype-object
+es6id: 21.2.5
+description: >
+  The RegExp prototype object does not have a [[RegExpMatcher]] internal slot
+info: |
+  The RegExp prototype object is an ordinary object. It is not a RegExp
+  instance and does not have a [[RegExpMatcher]] internal slot or any of the
+  other internal slots of RegExp instance objects.
+
+  21.2.5.2 RegExp.prototype.exec
+
+  1. Let R be the this value.
+  2. If Type(R) is not Object, throw a TypeError exception.
+  3. If R does not have a [[RegExpMatcher]] internal slot, throw a TypeError
+     exception.
+---*/
+
+assert.throws(TypeError, function() {
+  RegExp.prototype.exec('');
+});