Add some (commented) tests for 3.79

This needs working out how to load modules from within modules to work properly.

streams.rkt

@@ -780,6 +780,18 @@
                       (scale-stream y b))))
       y)))
 
+;; (module+ test
+;;   ;; Test: a=0, b=-1, y0=1, dy0=0: d^2/dx(cos(x)) = -cos(x).  Evaluate at pi -> -1
+;;   ;;       a=0, b=-1, y0=0, dy0=1: d^2/dx(sin(x)) = -sin(x).  Evaluate at pi -> 0
+
+;;   (check (close-to (stream-ref (solve-2nd 0 -1 (/ 3.1415 1000) 1 0) 1000)
+;;                    -1
+;;                    0.01))
+  
+;;   (check (close-to (stream-ref (solve-2nd 0 -1 (/ 3.1415 1000) 0 1) 1000)
+;;                    0
+;;                    0.01)))
+
 (module+ 3-79
   (#%require racket/local)
   (define (solve-2nd f dt y0 dy0)