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}