MDL-67317 behat: Ensure that on every scenario we have a session.

MDL-67317 behat: Ensure that on every scenario we have a session.

Before Mink 1.8.0, any call to getSession() used to auto-start
the session if it was not avaliable there. That always has been
an unsupported detail.

With https://github.com/minkphp/Mink/pull/705 new sessions aren't
auto-created anymore.

And we were relying on that.

So this is, simply about to explicitly create the new session
in our before_scenario stuff, to guarantee that it's always there.