Moreover, it is important to check that the primitive functions raise exceptions correctly. To give you an idea I have added unit tests for fx+ and fl+. Here's the unit test for fx+:
(check-exn type-exception? (lambda () (fx+ 1/2)))
(check-exn type-exception? (lambda () (fx+ 1/2 9)))
(check-exn type-exception? (lambda () (fx+ 9 1/2)))
(check-exn type-exception? (lambda () (fx+ 1/2 3 9)))
(check-exn type-exception? (lambda () (fx+ 3 1/2 9)))
(check-exn type-exception? (lambda () (fx+ 3 9 1/2)))
(check-exn fixnum-overflow-exception? (lambda () (fx+ ##max-fixnum 1)))
(check-exn fixnum-overflow-exception? (lambda () (fx+ ##min-fixnum -1)))