add missing unit tests file for dynamic scope

tests/dynamic.scm

@@ -0,0 +1,59 @@
+(define-macro (exec code)
+  (lips.evaluate code &(:use_dynamic true)))
+
+(test "dynamic: let"
+      (lambda (t)
+        (t.is (exec (begin
+                      (define (f)
+                        (let ((y 10))
+                          (+ x y)))
+                      (let ((x 10))
+                        (f))))
+              20)))
+
+(test "dynamic: function parameters"
+      (lambda (t)
+        (t.is (exec (begin
+                      (define (f y)
+                        (+ x y))
+                      (let ((x 10))
+                        (f 10))))
+              20)))
+
+(test "dynamic: set"
+      (lambda (t)
+        (t.is (exec (begin
+                      (define (f)
+                        (set! x (* x x)))
+                      (let ((x 10))
+                        (f)
+                        x)))
+              100)))
+
+;; https://www.reddit.com/r/lisp/comments/1943lg6/comment/khehb28/?context=3
+(test "dynamic: stak"
+      (lambda (t)
+        (t.is
+         (exec (begin
+                 (define (stak x y z)
+                   (stak-aux))
+
+                 (define (stak-aux)
+                   (if (not (< y x))
+                       z
+                       (let
+                           ((x (let ((x (1- x))
+                                     (y y)
+                                     (z z))
+                                 (stak-aux)))
+                            (y (let ((x (1- y))
+                                     (y z)
+                                     (z x))
+                                 (stak-aux)))
+                            (z (let ((x (1- z))
+                                     (y x)
+                                     (z y))
+                                 (stak-aux))))
+                         (stak-aux))))
+                      (stak 18 12 6)))
+         7)))