Add tests for objects created in the RelativeTimeFormat constructor.

The specification was changed in https://github.com/tc39/proposal-intl-relative-time/pull/79.

test/intl402/RelativeTimeFormat/constructor/constructor/options-proto.js

+// Copyright 2018 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-Intl.RelativeTimeFormat
+description: |
+    Checks that the RelativeTimeFormat constructor does not cause the
+    NumberFormat and PluralRules constructors to get properties off
+    Object.prototype through the options objects it creates.
+info: |
+    InitializeRelativeTimeFormat (relativeTimeFormat, locales, options)
+    20. Let nfOptions be ObjectCreate(null).
+    25. Let prOptions be ObjectCreate(null).
+features: [Intl.RelativeTimeFormat]
+---*/
+
+Object.defineProperties(Object.prototype, {
+  // NumberFormat & PluralRules
+  "localeMatcher": {
+    "get": function() {
+      throw new Test262Error("Should not call getter on Object.prototype: localeMatcher");
+    },
+  },
+
+  "minimumIntegerDigits": {
+    "get": function() {
+      throw new Test262Error("Should not call getter on Object.prototype: minimumIntegerDigits");
+    },
+  },
+
+  "minimumFractionDigits": {
+    "get": function() {
+      throw new Test262Error("Should not call getter on Object.prototype: minimumFractionDigits");
+    },
+  },
+
+  "maximumFractionDigits": {
+    "get": function() {
+      throw new Test262Error("Should not call getter on Object.prototype: maximumFractionDigits");
+    },
+  },
+
+  "minimumSignificantDigits": {
+    "get": function() {
+      throw new Test262Error("Should not call getter on Object.prototype: minimumSignificantDigits");
+    },
+  },
+
+  "maximumSignificantDigits": {
+    "get": function() {
+      throw new Test262Error("Should not call getter on Object.prototype: maximumSignificantDigits");
+    },
+  },
+
+  // NumberFormat
+  "style": {
+    "get": function() {
+      throw new Test262Error("Should not call getter on Object.prototype: style");
+    },
+  },
+
+  "currency": {
+    "get": function() {
+      throw new Test262Error("Should not call getter on Object.prototype: currency");
+    },
+  },
+
+  "currencyDisplay": {
+    "get": function() {
+      throw new Test262Error("Should not call getter on Object.prototype: currencyDisplay");
+    },
+  },
+
+  "useGrouping": {
+    "get": function() {
+      throw new Test262Error("Should not call getter on Object.prototype: useGrouping");
+    },
+  },
+
+  // PluralRules
+  "type": {
+    "get": function() {
+      throw new Test262Error("Should not call getter on Object.prototype: type");
+    },
+  },
+});
+
+new Intl.RelativeTimeFormat();