Remove invalid UTF-8 characters from test output

tools/testrunner/local/commands.py

 # Copyright 2012 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 93 matching lines @@
   timeout_result = [False]
 
   timer = Timer(timeout, kill_process, [process, timeout_result])
   timer.start()
   stdout, stderr = process.communicate()
   timer.cancel()
 
   return output.Output(
       process.returncode,
       timeout_result[0],
-      stdout,
-      stderr,
+      stdout.decode('utf-8', 'replace').encode('utf-8'),
+      stderr.decode('utf-8', 'replace').encode('utf-8'),
       process.pid,
   )
 
 
 def Execute(args, verbose=False, timeout=None):
   args = [ c for c in args if c != "" ]
   return RunProcess(verbose, timeout, args=args)