factored out a common pattern in tests

pkgs/htdp-pkgs/htdp-test/2htdp/tests/test-aux.rkt

@@ -0,0 +1,17 @@
+#lang racket/gui
+
+(require rackunit)
+
+(provide
+  ;; syntax
+  ;; (testing e ...) creates a top-level test in its own eventspace that
+  ;; shuts down after the tests are run 
+  testing
+  (all-from-out rackunit))
+
+;; -----------------------------------------------------------------------------
+
+(define-syntax-rule
+  (testing e ...)
+  (begin ;; module+ test
+    (parameterize ((current-eventspace (make-eventspace))) e ...)))