[mjsunit] Speed up array-concat.js test by not checking the entire array

This test creates an array that is 500000 elements long. Calling
assertEquals on this with another array is really slow, especially on
simulator runs. Most of this array is empty, only the first few elements
and last few elements contain meaningful items, so we check those
specific indices.

On a local run this test goes from ~250s (--jitless) or ~111s to <1s.

out/arm64.build/d8 --test test/mjsunit/mjsunit.js
test/mjsunit/array-concat.js --random-seed=1 --nohard-abort
--testing-d8-test-runner [--jitless]

(using --jitless makes the test even slower)

Bug: v8:7783
Change-Id: I660d3a9f1b3fe3afaa58fce28f493641059ba226
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2504853
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70825}

test/mjsunit/array-concat.js

@@ -225,15 +225,31 @@ assertEquals([undefined,2,1,3,"X"], r2);
 
 // Make first array change length of second array massively.
 arr2.length = 2;
+var largeLength = 500000;
 Object.defineProperty(arr1, 0, {get: function() {
-      arr2[500000] = "X";
+      arr2[largeLength] = "X";
       return undefined;
     }, configurable: true})
 var r3 = [].concat(arr1, arr2);  // [undefined,2,1,3,"X"]
 var expected = [undefined,2,1,3];
-expected[500000 + 2] = "X";
-
-assertEquals(expected, r3);
+var initialLen = expected.length;
+expected[largeLength + 2] = "X";
+
+var numElementsToCheck = 10;
+
+// Checking entire massive array is too slow, so check:
+var slicesToCheck = [
+  // - the first few elements,
+  {start: 0, end: initialLen},
+  // - arbitrary number of elements past the first few elements,
+  {start: initialLen, end: initialLen + numElementsToCheck},
+  // - arbitrary number of elements in the middle of the array
+  {start: largeLength / 2, end: largeLength / 2 + numElementsToCheck},
+  // - last few elements
+  {start: largeLength, end: largeLength + 3}];
+for (const {start, end} of slicesToCheck) {
+  assertEquals(expected.slice(start, end), r3.slice(start, end));
+}
 
 var arr3 = [];
 var trace = [];