Fix selenium chrome options so files are downloaded to tmp/downloads in system tests
as posted in https://github.com/SeleniumHQ/selenium/issues/5292#issuecomment-388810585
changing the hash key from
goog:chromeOptions fixes the issue.
Unfortunately it is unclear to me if we have to retain the old 'chromeOptions'
key for windows support and I have no way to try this out.
https://chromedriver.chromium.org/capabilities also uses the
'goog:' prefix in examples without mentioning any specific platform so I
think it will just work.