[autotest] Ensure that we clear /tmp/uptime-login-prompt-ready before starting a new test

This change should help ensure that we don't think the login prompt is up and ready for input before it's really up and ready.

BUG=chrome-os-partner:711
TEST=run BVT

Change-Id: I17ef715ea6527757c124f8d53c6ca1320cb1e140

Review URL: http://codereview.chromium.org/3213006
1 file changed