servo_test: Fix a bug in wait_for_client_offline

BUG=chromium-os:34351
TEST=Test passed

Change-Id: Ib494d77400726754c9746b69491daf325148f123
Signed-off-by: Vic Yang <victoryang@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/33291
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
diff --git a/server/cros/servo_test.py b/server/cros/servo_test.py
index 1187f8f..0784240 100644
--- a/server/cros/servo_test.py
+++ b/server/cros/servo_test.py
@@ -270,6 +270,7 @@
         """
         # Wait for the client to come offline.
         while timeout > 0 and self._ping_test(self._client.ip, timeout=1):
+            time.sleep(1)
             timeout -= 1
         assert timeout, 'Timed out waiting for client offline.'
         logging.info('Server: Client machine is offline.')