Fix keyboard interrupt in test driver.

Drain the queues to prevent failures when queues are garbage collected. Fails when interrupting test262 otherwise.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/286973004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21334 ce2b1a6d-e550-0410-aec6-3dcde31c8c00