Add delay time before retrying when downloading the test file in wireless.py.

There's short time delay between connecting to the AP and getting the IP
address. Thus, when running this test, it is possible to fail because
all the retries happened before getting the IP address.

TODO(littlecvr): This problem should be solved by probing the IP address
                 of the wireless interface before any download starts to
                 take action. Although it is very reasonable to add some
                 delay before every retries.

BUG=none
TEST=manually tested

Change-Id: Id12c4c76687d1633d603950e20d71dd96321aa0d
Reviewed-on: https://chromium-review.googlesource.com/177175
Reviewed-by: Cheng-Yi Chiang <cychiang@chromium.org>
Commit-Queue: Mao Huang <littlecvr@chromium.org>
Tested-by: Mao Huang <littlecvr@chromium.org>
1 file changed