[functional-tests] move swap! to (utils)

functional-tests/utils.scm

@@ -1,7 +1,8 @@
 (library
   (utils)
   (export inc!
-          dec!)
+          dec!
+          swap!)
   (import (rnrs))
 
   (define-syntax inc!
@@ -13,4 +14,11 @@
     (syntax-rules ()
       ((_ v) (set! v (- v 1)))
       ((_ v n) (set! v (- v n)))))
+
+  (define-syntax swap!
+    (syntax-rules ()
+      ((_ x y)
+       (let ((tmp x))
+        (set! x y)
+        (set! y tmp)))))
   )