Add tests for Array.prototype.sort stability

tc39/ecma262#1340

test/built-ins/Array/prototype/sort/stability-11-elements.js

@@ -0,0 +1,37 @@
+// Copyright (C) 2018 Mathias Bynens. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-array.prototype.sort
+description: >
+  Stability of Array.prototype.sort for an array with 11 elements.
+info: |
+  The sort is required to be stable (that is, elements that compare equal
+  remain in their original order).
+  The array length of 11 was chosen because V8 used an unstable
+  QuickSort for arrays with more than 10 elements until v7.0 (September
+  2018). https://v8.dev/blog/array-sort
+---*/
+
+const array = [
+  { name: 'A', rating: 2 },
+  { name: 'B', rating: 3 },
+  { name: 'C', rating: 2 },
+  { name: 'D', rating: 4 },
+  { name: 'E', rating: 3 },
+  { name: 'F', rating: 3 },
+  { name: 'G', rating: 4 },
+  { name: 'H', rating: 3 },
+  { name: 'I', rating: 2 },
+  { name: 'J', rating: 2 },
+  { name: 'K', rating: 2 },
+];
+assert.sameValue(array.length, 11);
+
+// Sort the elements by `rating` in descending order.
+// (This updates `array` in place.)
+array.sort((a, b) => b.rating - a.rating);
+
+const reduced = array.reduce((acc, element) => acc + element.name, '');
+const isProbablyStable = reduced === 'DGBEFHACIJK';
+assert(isProbablyStable);

test/built-ins/Array/prototype/sort/stability-5-elements.js

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+// Copyright (C) 2018 Mathias Bynens. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-array.prototype.sort
+description: >
+  Stability of Array.prototype.sort for an array with 5 elements.
+info: |
+  The sort is required to be stable (that is, elements that compare equal
+  remain in their original order).
+---*/
+
+const array = [
+  { name: 'A', rating: 2 },
+  { name: 'B', rating: 3 },
+  { name: 'C', rating: 2 },
+  { name: 'D', rating: 3 },
+  { name: 'E', rating: 3 },
+];
+assert.sameValue(array.length, 5);
+
+// Sort the elements by `rating` in descending order.
+// (This updates `array` in place.)
+array.sort((a, b) => b.rating - a.rating);
+
+const reduced = array.reduce((acc, element) => acc + element.name, '');
+const isProbablyStable = reduced === 'BDEAC';
+assert(isProbablyStable);