Fix subtraction, convert tests to rackunit

rint.rkt

@@ -21,6 +21,7 @@
     [(Int n) n]
     [(Prim 'read '()) (read-fixnum)]
     [(Prim '- (list e)) (fx- 0 (interp-exp e))]
+    [(Prim '- (list e1 e2)) (fx- (interp-exp e1) (interp-exp e2))]
     [(Prim '+ (list e1 e2)) (fx+ (interp-exp e1) (interp-exp e2))]))
 
 (define (interp-RInt p)

test-rint.rkt

@@ -15,11 +15,16 @@
 (define test-rint-tests
  (test-suite
   "RInt interpretation tests"
-  (test-case 
+  (test-case
    "program with input"
    (check-equal?
     -5
     (with-input-from-num-list '(3)
-       (lambda () (interp-RInt program)))))))
+       (lambda () (interp-RInt program)))))
+  (test-case 
+   "simple difference"
+   (check-equal?
+    -3
+    (interp-RInt (Program '() (Prim '- (list (Int 5) (Int 8)))))))))