MDL-43839 is integrated the Behat tests will use Clean by default. Currently they don't work. Initial errors are because the log in link is hidden within an icon so it isnt found so logging in fails. It is likely that there are other problems waiting to be discovered.
One possible partial fix is at https://github.com/andyjdavis/moodle/commit/d407890e0c3d2eb101c6f3e3f4eca179a6c30539
See these comments for previous discussion.