Order by last_update when not searching by keyword

In order to match the finder-api behaviour that rummager is taking over,
we want to order results when filtering by the date they were last
updated.

This last_update field, once added to rummager in the schema and
permitted sort fields, and provided by the publishing applications, will
allow this behaviour to be maintained.

See alphagov/rummager#294 and alphagov/specialist-publisher#250
https://github.com/alphagov/finder-frontend/commit/0d38d44d6cbad57b58efad62aad61aa129aeb125

The test suite was written expecting the order to match the finder API mocked data, it appears. But when using Rummager now the mocks for that are ordered differently. I think.