Make format weighting unit test less fragile
Make format weighting unit test less fragile

Replace assertion of full Elasticsearch query with assertions about a
sample of format weightings.

This makes the test less fragile: adding extra format weightings should
not make the test fail, since it's just a configuration change.

This will also make it easier to refactor the format weighting code.

alphagov/rummager,alphagov/rummager