Add some behat tests for customised functionality

[robbieaverill]

We should add some behat tests for the customised functionality in the theme. For example:

• Navigation tabbing with keyboard
• Dropdowns in navigation for sub-pages
• Mobile view for navigation
• Mobile and desktop search (although this would require Solr to could be too hard basket)
• Filtering CWP news and events by date or tag
• Bootstrap form validation plugin (form.js)
• Sitemap: expanding sub-nodes