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 minkphp/Mink#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.

Author: stronk7

lib/tests/behat/behat_hooks.php

@@ -307,6 +307,9 @@ public function before_scenario(BeforeScenarioScope $scope) {
         $driverexceptionmsg = 'Selenium server is not running, you need to start it to run tests that involve Javascript. ' . $moreinfo;
         try {
             $session = $this->getSession();
+            if (!$session->isStarted()) {
+                $session->start();
+            }
         } catch (CurlExec $e) {
             // Exception thrown by WebDriver, so only @javascript tests will be caugth; in
             // behat_util::check_server_status() we already checked that the server is running.