test: change test expectation for string decoder

V8 has changed their invalid UTF-8 handling. See
https://chromium-review.googlesource.com/c/v8/v8/+/671020 for more info

PR-URL: https://github.com/nodejs/node/pull/16271


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Myles Borins <myles.borins@gmail.com>