adjust plai's test facility to print test failures to stderr

instead of stdout (in addition to making errors red in drracket,
this will also make some of the existing test suites more
drdr friendly)

collects/plai/scribblings/plai.scrbl

@@ -124,7 +124,8 @@ PLAI Scheme provides the following syntactic forms for testing.
 @defform/subs[(test result-expr expected-expr)()]{
 
 If @racket[_result-expr] and @racket[_expected-expr] evaluate to the same
-value, @racket[_result-value], the test prints
+value, @racket[_result-value], the test prints the following expression
+to @racket[(current-error-port)]:
 
 @racketresultfont{(good result-expr result-value expected-value location)}.
 

collects/plai/test-harness.rkt

@@ -69,9 +69,8 @@
     (set! plai-all-test-results (cons result plai-all-test-results))
     (when print?
       (if (abridged-test-output)
-          (apply printf "(~s ~v ~v)" result)
-          (apply printf "(~s ~s ~v ~v ~s)" result))
-      (newline))
+          (apply eprintf "(~s ~v ~v)\n" result)
+          (apply eprintf "(~s ~s ~v ~v ~s)\n" result)))
     (when (and halt-on-errors? error?)
       (raise (make-exn:test (string->immutable-string (format "test failed: ~s" result))
                             (current-continuation-marks))))))

collects/tests/plai/test-harness.rkt

@@ -7,7 +7,13 @@
   [id (s symbol?)])
 
 (define-syntax-rule (->string e)
-  (regexp-replace "line [0-9]+" (with-output-to-string (λ () e)) "line ??"))
+  (regexp-replace "line [0-9]+" (with-err-output-to-string (λ () e)) "line ??"))
+
+(define (with-err-output-to-string thunk)
+  (define sp (open-output-string))
+  (parameterize ([current-error-port sp])
+    (thunk))
+  (get-output-string sp))
 
 (define (go catch? errors? abridged?)
   (eli:test