Go back in the browser history and assert the previous page came back without a full page reload, as Gatsby's router handles it on popstate.
Examples
When I go back without a full page reload
When we go back without a full page reload
And I go back without a full page reload
When go back without a full page reload
And we go back without a full page reload
Pattern
When(/^(I |we )*go back without a full page reload$/, …)
Part of @webship-js/gatsby-steps (github.com/webship/gatsby-steps), a step pack for Webship-js 2.0.x.