Don't create multiple browser sessions per scenario

This is quite an obscure bug (although bug is maybe not the right term). Normally you will never be bothered by this unless you are using saucelabs and switch personas within a scenario (although it can be triggered by using 'Given a browser' in a background/scenario and then using a persona in a scenario). The result of this is that saucelabs has has several sessions started for the same scenario and can't keep track of more than one, so they will eventually time out or terminate and it then looks a mess.