[racket] testing impure stuff

Hi there,
I just wrote a file duplicate finder where I'm not quite sure how to
build up my test cases.

The problem is that most of my test cases are impure. They rely upon a
directory layout.

What is the best way to do this? Should I create a directory structure
containing most (or better all) of my test cases, and then base my test
cases upon the existing structure?

Or perhaps even better create my directory structure on the fly and
build my test cases upon this?


How do other people create test cases for impure situations?



-- 
Manfred