Make sure nobody is logged in to the administrator panel: clear the browser's cookies and web storage, load /administrator again, and assert the login form (#form-login) is showing.
Examples
Given I am not logged in
Given we are not logged in
And not logged in
Given I am not logged in
And we are not logged in
Pattern
Given(/^(I am |we are )*not logged in$/, …)
Part of @webship-js/joomla-steps (github.com/webship/joomla-steps), a step pack for Webship-js 2.0.x.