Add "index" integer coercion test with functional replacer

test/built-ins/RegExp/prototype/Symbol.replace/result-coerce-index-undefined.js

@@ -0,0 +1,41 @@
+// Copyright (C) 2020 Alexey Shvayka. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-regexp.prototype-@@replace
+description: >
+  Integer coercion of "index" property of the value returned by RegExpExec.
+  (undefined value)
+info: |
+  RegExp.prototype [ @@replace ] ( string, replaceValue )
+
+  [...]
+  14. For each result in results, do
+    [...]
+    e. Let position be ? ToInteger(? Get(result, "index")).
+    [...]
+
+  ToInteger ( argument )
+
+  1. Let number be ? ToNumber(argument).
+  2. If number is NaN, return +0.
+features: [Symbol.toPrimitive, Symbol.replace]
+---*/
+
+var index = {};
+var toPrimitiveHint;
+index[Symbol.toPrimitive] = function(hint) {
+  toPrimitiveHint = hint;
+};
+
+var r = /./;
+r.exec = function() {
+  return {
+    length: 1,
+    0: 'a',
+    index: index,
+  };
+};
+
+assert.sameValue(r[Symbol.replace]('ab', '$`'), 'b');
+assert.sameValue(toPrimitiveHint, 'number');