Fix test262 harness to purge old data directory.

Fix test262 harness to purge old data directory.

R=jkummerow@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=11324

[Michael Starzinger, 2012-04-16 08:48:29]

This just renames any old test data directory after a new archive is downloaded.
There are several reasons why to do it this way:
* Preserve data directories the user might have around.
* On buildbots any untracked directory will be deleted before the next run by
gclient.
* It is hard(TM) to recursively remove directories on Windows buildbots, better
let gclient do it.

If you have a more elegant solution, let me know. But I want to keep the
"DownloadData" routine as simple as possible. So I am hesitant to adding a stamp
file or something like that.

[Jakob Kummerow, 2012-04-16 08:55:45]

lgtm