Prettyprint symbols in mjsunit tests.

R=jochen
BUG=

Review URL: https://codereview.chromium.org/1424793004

Cr-Commit-Position: refs/heads/master@{#31595}

test/mjsunit/mjsunit.js

@@ -131,6 +131,7 @@ var assertUnoptimized;
       case "boolean":
       case "undefined":
       case "function":
+      case "symbol":
         return String(value);
       case "object":
         if (value === null) return "null";