This CL makes the test stricter by requiring specific string shapes;
before it was possible to skip verification if strings were
short-circuited, which I believe is no longer possible due to thin
strings.

I also added a regression test for the linked bug, which requires
a String.p.split call on a two-byte-in-one-byte string with an empty
string separator argument.

Bug: chromium:1088179
Change-Id: Ibb3180afe612a64fcf6a506d18bbc415840526a8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2228609Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68128}