add tests to check if the code detects invalid/malicious bytes in a UTF-8

[printfdebugging]

• use https://www.rfc-editor.org/info/rfc3629/#ref-UNICODE for reference.
• nothing seems obviously wrong with the code as it is now, but no extra effort was put in on the security aspects of it, so that needs some more attention.

[printfdebugging]

see https://www.unicode.org/versions/Unicode17.0.0/core-spec/chapter-3/#G27506. it defines what is a valid utf8 byte sequence should look like.