This pr refactor all usage of v8_str in unittests/ into NewString
method in test-utils.h

Change-Id: I8ffc456851488e8c050d7a38f5459aebfa3f314f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3813064Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: 王澳 <wangao.james@bytedance.com>
Cr-Commit-Position: refs/heads/main@{#82314}