smallpt: truncate result

Current test does some floating-point manipulations which can be non-deterministic
due to different platforms/architectures. This change truncates the result to
avoid potential stdout mismatch.

real/smallpt/smallpt.hs

@@ -222,5 +222,4 @@ main = do
   vec <- smallpt w h samples
   if mode == "img"
     then writeImage stdout (w, h) vec
-    else  hPutStr stdout (hashImage vec)
-
+    else hPutStr stdout (printf "%.6f" (read (hashImage vec) :: Double))

real/smallpt/smallpt.stdout

-42517.5950581666
\ No newline at end of file
+42517.595058