Assert the local-tasks block — the tab strip core builds from a module's *.links.task.yml — is, or is not, present. Matched by the primary-tabs-title label core's template references plus the .tabs classes, so it holds across base themes.
Examples
Then I should see the local tasks
And we should see the tabs
Then should see the primary tabs
And I should see the local tasks
Then we should not see the local tasks
Pattern
Then(/^(I |we )*should( not)* see the (?:primary )?(?:local tasks|tabs)$/, …)
Part of @webship-js/drupal-core-steps (github.com/webship/drupal-core-steps), a step pack for Webship-js 2.0.x.