Extend tests for Annex B String HTML methods (#627)

- Add tests for abrupt completion handling from ToString operations
- Add tests for method property descriptors
- Remove redundant assertion

test/annexB/built-ins/String/prototype/sup/B.2.3.14.js

@@ -21,4 +21,3 @@ assert.throws(TypeError, function() {
 assert.throws(TypeError, function() {
   String.prototype.sup.call(null);
 });
-assert.sameValue(String.prototype.sup.length, 0);

test/annexB/built-ins/String/prototype/sup/prop-desc.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-string.prototype.sup
+es6id: B.2.3.14
+description: Property descriptor for String.prototype.sup
+info: >
+    Every other data property described in clauses 18 through 26 and in Annex
+    B.2 has the attributes { [[Writable]]: true, [[Enumerable]]: false,
+    [[Configurable]]: true } unless otherwise specified.
+includes: [propertyHelper.js]
+---*/
+
+verifyNotEnumerable(String.prototype, 'sup');
+verifyWritable(String.prototype, 'sup');
+verifyConfigurable(String.prototype, 'sup');

test/annexB/built-ins/String/prototype/sup/this-val-tostring-err.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-string.prototype.sup
+es6id: B.2.3.14
+description: Abrupt completion when coercing "this" value to string
+info: |
+    B.2.3.2.1 Runtime Semantics: CreateHTML
+
+    1. Let str be ? RequireObjectCoercible(string).
+    2. Let S be ? ToString(str).
+---*/
+
+var thisVal = {
+  toString: function() {
+    throw new Test262Error();
+  }
+};
+
+assert.throws(Test262Error, function() {
+  String.prototype.sup.call(thisVal);
+});