Expand specialist_sectors field

The specialist sectors aren't yet being indexed, but when they are this
will the specialist_sectors field in search results.

The main reason for implementing this now is that we also want to store
example values in the SpecialistSectorsRegistry, so we need to create
that object first.
Support stubbing the metasearch index

Also, memoize the index stubs so that repeated calls to them within a
test return the same stub.
Functional test for sector expansion