[Ruby 3.4] Add tests for warning about mutating chilled Strings and String#+@

[trinistr]

From #1265.

String literals in files without a frozen_string_literal comment now emit a deprecation warning
when they are mutated.
These warnings can be enabled with -W:deprecated or by setting Warning[:deprecated] = true.
To disable this change, you can run Ruby with the --disable-frozen-string-literal
command line argument. [Feature #20205]

• String#+@ now duplicates when mutating the string would emit
  a deprecation warning, offered as a replacement for the
  str.dup if str.frozen? pattern.

Maybe this needs to be squashed? But they are separate things 🤷