Update XPath for search result highlighting
Update XPath for search result highlighting

The search result highlighting has changed from using the `<em>` element to the `<mark>` element.

https://github.com/alphagov/digitalmarketplace-search-api/pull/54

The commit updates the tests accordingly.
alphagov/digitalmarketplace-functional-tests,alphagov/digitalmarketplace-functional-tests,alphagov/digitalmarketplace-functional-tests