msunit/regress/regress-542823 is intended to test large object
allocation in Array.prototype.join, but to do so it has a pretty
inefficient way of first building a large array.

Speed-up this test by using Array.prototype.fill, call .join directly,
and make the whole thing an IIFE to avoid global loads.

Bug: v8:11060
Change-Id: I5906bcb6c65b10ec830b026cf1f24acb6d5e1aaf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2498681
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70755}