Behat functional tests

During WordCamp Rotterdam I worked on a small bug in WP-CLI. A bug I found roughly six months ago.
The bug itself was a one line fix.
I was asked to create functional tests using behat which I didn’t even heard off before.

So after checking the handbook and with help of a few others. It resulted in this pull request.
And now we wait.

Update: It was released as part of wp-cli-2.0.0