[string] Add additional ThinString test

Add a test for the case where SlicedStrings of ThinStrings are looked up
in the string table, testing the path that the original string's length
differs from the actual string's length.

Bug: chromium:1309767
Change-Id: I909c64397bf28ec33c3324d94882fbfe81ac4109
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3549837


Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79610}