🧪 Simplify error message formatting in test cases

[coderabbitai]

As discussed in PR #769 comment, the error messages in test cases can be simplified by removing the unnecessary transformation using strings.Join(strings.Split(...)). This was initially implemented because error messages used to be atoms but are now strings (lists of characters), and the change was made to update the tests without too much hassle.

Refactoring the error messages to use direct string literals will improve readability and maintainability.

Requested by @ccamel.