Click the "Log out" link in the administrator panel's user menu and wait for the login form to reappear.
Examples
When I log out of the administrator panel
When we log out of the administrator panel
And I log out of the administrator panel
When I log out of the admin panel
And we log out of the admin panel
Pattern
When(/^(I |we )*log out of the admin(?:istrator)* panel$/, …)
Part of @webship-js/joomla-steps (github.com/webship/joomla-steps), a step pack for Webship-js 2.0.x.