Navigate to the login screen at /web/login. The screen ships with the base "web" module, so every Odoo site has it.
Examples
Given I am on the login page
Given we are on the login page
Given I am on the sign in page
And on the sign-in page
Given I am on the web/login page
Pattern
Given(/^(I am |we are )*on the (?:sign[- ]?in|web\/login|login) page$/, …)
Part of @webship-js/odoo-steps (github.com/webship/odoo-steps), a step pack for Webship-js 2.0.x.