Flush caches through the "Clear cache" item of the toolbar: "frontend" is "Flush frontend caches" (pages and their content), "all" is "Flush all caches" (the system caches too, administrators only). The step waits for the request to finish.
Examples
When I flush the frontend caches
When I flush all caches
When we flush the frontend caches
And I flush all the caches
When flush the frontend caches
Pattern
When(/^(I |we )*flush (the frontend|all|all the) caches$/, …)
Part of @webship-js/typo3-steps (github.com/webship/typo3-steps), a step pack for Webship-js 2.0.x.