Add test for NaN in OrdinaryDefineOwnProperty (#648)

These semantics are currently contested, but the test reflects the
intended behavior of the latest version of the specification.

test/built-ins/Object/internals/DefineOwnProperty/nan-equivalence.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-ordinary-object-internal-methods-and-internal-slots-defineownproperty-p-desc
+es6id: 9.1.6
+description: >
+  Compares "value" field using SameValue algorithm (thereby ignoring distinct
+  NaN values)
+info: |
+  1. Return ? OrdinaryDefineOwnProperty(O, P, Desc).
+
+  9.1.6.1 OrdinaryDefineOwnProperty
+
+  1. Let current be ? O.[[GetOwnProperty]](P).
+  2. Let extensible be the value of the [[Extensible]] internal slot of O.
+  3. Return ValidateAndApplyPropertyDescriptor(O, P, extensible, Desc,
+     current).
+
+  9.1.6.3 ValidateAndApplyPropertyDescriptor
+
+  [...]
+  4. Return true, if every field in Desc also occurs in current and the value
+     of every field in Desc is the same value as the corresponding field in
+     current when compared using the SameValue algorithm.
+features: [Float64Array, Uint8Array]
+includes: [nans.js]
+---*/
+
+var float = new Float64Array(1);
+var ints = new Uint8Array(float.buffer);
+var length = distinctNaNs.length;
+var idx, jdx, subject, first, second;
+function byteValue(value) {
+  float[0] = value;
+  return ints[0] + (ints[1] << 8) + (ints[2] << 16) + (ints[3] << 32) +
+    (ints[4] << 64) + (ints[5] << 64) + (ints[6] << 128) + (ints[7] << 256);
+}
+
+/**
+ * Iterate over each pair of distinct NaN values (with replacement). If two or
+ * more suitable NaN values cannot be identified, the semantics under test
+ * cannot be verfied and this test is expected to pass without evaluating any
+ * assertions.
+ */
+for (idx = 0; idx < length; ++idx) {
+  for (jdx = 0 ; jdx < length; ++jdx) {
+    first = distinctNaNs[idx];
+    second = distinctNaNs[jdx];
+    if (byteValue(first) === byteValue(second)) {
+      continue;
+    }
+
+    subject = {};
+    subject.prop = first;
+    subject.prop = second;
+
+    assert.notSameValue(
+      byteValue(subject.prop),
+      byteValue(second),
+      'Property value was not re-set'
+    );
+  }
+}