Add support for JUnit compatible XML output in the new test runner.

Implement a new output method in run-tests.py which
outputs JUnit compatible XML status information.

BUG=

Review URL: https://codereview.chromium.org/13813003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14217 ce2b1a6d-e550-0410-aec6-3dcde31c8c00