racket-mock 
Mocking library for Racket RackUnit testing.
raco pkg install jack-mock
Documentation: mock
This library allows for easy construction of mocks, which are "fake" implementations of functions that record calls made to them for testing.
Currently unstable, API changes may occur.
Example:
(require mock)
(define/mock (displayln-twice v)
([displayln (void-mock)]) ; in the test submodule, calls a mock instead of displayln
(displayln v)
(displayln v))
(displayln-twice "sent to real displayln")
(mock? displayln) ; #f
(mock-num-calls displayln) ; error - displayln isn't a mock
(module+ test
(displayln-twice "sent to mock displayln")
(mock? displayln) ; #t
(mock-num-calls displayln)) ; 2
