[#64241148] Define Cucumber tests for static mirrors

This PR:
- defines Cucumber tests for use with Smokey on the mirrors
- adds new step definition for `seeing services and information on homepage`

We don't cache using Varnish in static, but the feature file will force cachebusts. This allows the file to be somewhat futureproof in case we do start using Varnish to cache in future. Search is a separate application and will not boot when static is used, therefore ensure that we are returning 503s and `This page cannot be found` text appears on the page.