Assert the status report (/admin/reports/status) counts no errors or no warnings, or that it does. The report is read with the current session, so the page on screen is left alone. A failure names the entries, which is what a person opens the report for.
Examples
Then the status report should have no errors
And the status report should not have any errors
Then the status report should have no warnings
And the status report should have errors
Then the status report should have warnings
Pattern
Then(/^the status report should (have no|not have any|not have|have) (errors|warnings)$/, …)
Part of @webship-js/drupal-core-steps (github.com/webship/drupal-core-steps), a step pack for Webship-js 2.0.x.