Listen to your feelings ;-). The tests in t/ should test your code and not do the work of the code itself.

Tests should be an independent instance that declares your code as correct, even after you changed parts of it or it is run on a different platform or your operating system is a new version. The important word is "independent". In your example you would write a test to check whether a missing directory produces an error message. If this test fails you know that that line doesn't work anymore in your script

Note it makes not much sense adding tests to old applications, unless you plan to rewrite, enhance or refactor the application, or adapt it to new environments