Extend string_concat() and string_join() to test combinations of strings with variables

[gnysek]

Description

There was recently an issue, in which string_concat, and string_join, were failing under very specific condition - if penultimate argument was a variable name, and last argument was a literal string - https://github.com/YoYoGames/GameMaker-Bugs/issues/6480 .

Basic idea is to extend BasicStringTestSuite for those functions (and their _ext versions, to include such specific cases too.

Steps To The Task

• [x] string_concat, string_concat_ext - 1st arg is variable, 2nd arg is string
• [x] string_concat, string_concat_ext - 1st arg is string, 2nd arg is variable
• [x] string_join, string_join_ext - 2nd arg is variable, 3rd arg is string
• [x] string_join, string_join_ext - 2nd arg is string, 3rd arg is variable

[gnysek]

@YYDan there's issue with contributing:

3. Back on your issue in our repository, set the branch in the right-hand side to be a new branch back on your own fork

Nothing is clickable for users outside of YYG in right side panel, that's how it looks on my side:

[YYDan]

Yeah, I am looking into a few issues for external contributors at the moment, but thanks for letting me know.