fix a race-condition in port finding: in ava Math.random() will get the same result in the child process?

fix CircleCI issue: https://discuss.circleci.com/t/docker-builds-are-failing-on-circleci/5267/6?u=zixia

selenium find `chromedriver.exe` by default under win32, so we have to specify the `chromedriver`.path in order to make it work with npm module `chromedriver`