BUG Fixing logout issues with steps

stepIAmNotLoggedIn() would previously just kill the test
session, but this has the side effect of losing the current testsession
temporary database, so this causes steps of logging out to fail the
rest of the steps in a scenario.

See https://github.com/silverstripe-labs/silverstripe-behat-extension/pull/12

src/SilverStripe/BehatExtension/Context/LoginContext.php

@@ -110,7 +110,8 @@ class LoginContext extends BehatContext
      */
     public function stepIAmNotLoggedIn()
     {
-        $this->getSession()->reset();
+        $c = $this->getMainContext();
+        $this->getSession()->visit($c->joinUrlParts($c->getBaseUrl(), 'Security/logout'));
     }
 
     /**