Assert the current page is, or is not, the core 404 template, by checking the page heading for "404" (rendered by addons/http_routing's http_routing.404 template on every Odoo site).
Examples
Then I should be on the Odoo 404 page
And I should be on the not found page
Then we should be on the Odoo 404 page
And should not be on the 404 page
Then I should land on the Odoo 404 page
Pattern
Then(/^(I |we )*should( not)* (?:be on|land on) the (?:Odoo )?(?:404|not found|page not found) page$/, …)
Part of @webship-js/odoo-steps (github.com/webship/odoo-steps), a step pack for Webship-js 2.0.x.