Adds Rummager test helpers module

Adds stubbing helper for Rummager unified search used to retrieve
services and information data for an org.

Story: https://www.pivotaltracker.com/story/show/75026206
Merge pull request #173 from alphagov/add-rummager-test-helpers

Adds Rummager test helpers module

https://www.pivotaltracker.com/story/show/75026206