Log out through the log out address of the site (/user/logout) and check the session ended.
Examples
When I log out of the site
When we log out of the site
And I log out of the site
When I sign out of the site
And we sign out of the site
Pattern
When(/^(I |we )*(?:log|sign) out of the site$/, …)
Part of @webship-js/backdrop-cms-steps (github.com/webship/backdrop-cms-steps), a step pack for Webship-js 2.0.x.