NOTE: how to test Racket GUI preferences

In Racket GUI, framework/preferences is a built-in module that allows you to set and get configuration for your GUI. However, invoking preferences:set-default twice is illegal, this usually happened when you are trying to test the code using preferences:set / preferences:get. Hence, you will need preferences:new-layer and preferences:current-layer, the following is an example from sauron.

(module+ test
  (require rackunit)
  (define test-layer (preferences:new-layer (preferences:current-layer)))
  (parameterize ([preferences:current-layer test-layer])
    (preferences:set-default 'current-project (current-directory) path-string?)

    (check-equal? (run "ls") (void))))