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

client/bin/site_login.py

Index: client/bin/site_login.py
diff --git a/client/bin/site_login.py b/client/bin/site_login.py
index 2eff093ca8da9e62cc505ed643876506319ca7bc..9df2a54a9c7397ae521ab1c3920bc86fe480a8ee 100644
--- a/client/bin/site_login.py
+++ b/client/bin/site_login.py
@@ -291,12 +291,13 @@ def refresh_login_screen(timeout=_DEFAULT_TIMEOUT):
     """
     if logged_in():
         raise UnexpectedCondition('Already logged in')
+    wait_for_browser()
+    wait_for_login_prompt()
     try:
       os.unlink(chromeos_constants.LOGIN_PROMPT_READY_MAGIC_FILE)
     except OSError, e:
       if e.errno != errno.ENOENT:
         raise e
-    wait_for_browser()
     nuke_process_by_name(chromeos_constants.BROWSER, with_prejudice=True)
     wait_for_browser()
     wait_for_login_prompt()