a8n: Add tests for campaign type to remove credentials

Created by: mrnugget

I decided before I add support for replaceWith and the optional-changeset-body-additions-per-campaign-type that are part of RFC 73, I'll add some tests.

Don't be scared of the diff here and look at it commit wise: the first commit exposes fields on graphqlbackend types so that I can fake the search from the outside, the second commit adds the tests.

I'm actually pretty now that I've invested the time to fake the search. Took a lot of fiddling and digging around. I predict (since I already had that locally) that next we need to expose the graphqalbackend.lineMatches, once we need the specific search result locations. And that should be easy with this setup. We'd only need to expose lineMatches.