Press the node form's Save button in a scenario that expects the form not to go through: an empty required field, a value the browser rejects. The step passes whether or not the form was submitted; follow it with "the "..." field should block submission as required", or with an error message assertion when the server is the one that refuses.
Examples
When I try to save the content
And I try to save the content item
When we try to save the content
And I try to save the node
When we try to save the node
Pattern
When(/^(I |we )*try to save the (?:content|content item|node)$/, …)
Part of @webship-js/drupal-core-steps (github.com/webship/drupal-core-steps), a step pack for Webship-js 2.0.x.