Fix Intermittent Selenium Test Failures

[chenba]

We are still having issues with the Selenium tests. It appears to have something to do with detecting and dismissing the onboarding step. With #4277, the tests now will quit and re-open Firefox, which will increase the probability of encountering a failure.

[chenba]

I'm re-opening this because we've been experiencing these failures quite frequently this week.

Here is what I know about these failures:

• "should take a shot" (the second one) and "should not display the Save button" (the last one) are the tests that time out
• the page action icon is not displayed when it happens

See this comment about enabling trace logs: https://github.com/mozilla-services/screenshots/blob/master/test/test.js#L27. Although the failures are harder to come by once that's enabled! Maybe it's because the timing's affected once that's on?

Another thing we could do with the tests is convert the promise chains to async/await. There are currently two tests using async/await. It won't help with this issue, but it'd make reading and understanding the code easier, I hope.