[test262] Add a new staging test for Array.p.flatMap

Bug: v8:14306
Change-Id: Ibc7a04288a0c31bfcf601aa864e02b6c067045c2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/5259306
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#92174}

test/staging/built-ins/Array/prototype/flatMap/callback-with-side-effects.js

@@ -0,0 +1,28 @@
+// Copyright (C) 2024 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-array.prototype.flatMap
+description: >
+  Array.prototype.flatMap is given a callback that modifies the array being
+  iterated.
+includes: [compareArray.js]
+---*/
+
+(function TestGrow() {
+  let array = [0,1,2,3];
+  function f(e) {
+    array[4] = 42;
+    return e;
+  }
+  assert.compareArray(array.flatMap(f), [0,1,2,3]);
+})();
+
+(function TestShrink() {
+  let array = [0,1,2,3];
+  function f(e) {
+    array.length = 3;
+    return e;
+  }
+  assert.compareArray(array.flatMap(f), [0,1,2]);
+})();