Make the array elements in msunit/regress/regress-542823 larger, so that
it takes fewer of them to force the joined string to go into large
object space. Also, set the array's size dynamically based on the
maximum non-large object size, rather than having a fixed magic "large
enough" size, and verify that the resulting joined string is indeed in
LO space.

This reduces the runtime of this test under slow_path and gc-stress from
minutes to seconds.

Bug: v8:11060
Change-Id: I51d960b6a3e052199f50c1a6ba6fbce1b6d1ae38
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2498689
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70762}