Fixed some memory leaks in unit tests.

test/cctest/test-utils.cc

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 156 matching lines @@
     int n = OS::SNPrintF(Vector<char>(buffer.start(), i), "%s", s);
     CHECK(n <= i);
     CHECK(n == length || n == -1);
     CHECK_EQ(0, strncmp(buffer.start(), s, i - 1));
     CHECK_EQ(kMarker, buffer[i]);
     if (i <= length) {
       CHECK_EQ(i - 1, strlen(buffer.start()));
     } else {
       CHECK_EQ(length, strlen(buffer.start()));
     }
+    buffer.Dispose();
   }
 }