Add AAIB and IDF schemas to TestHelpers::FinderApi

Previously, the finder_api_has_schema method would either default to
using its internal CMA case schema (which was outdated) or allow a
schema to be passed in.

Given that the test helpers already had a fixture for CMA cases, I
decided to add it here rather than add *another* place that needs
schemas included.
Merge pull request #181 from alphagov/add-schemas-to-test-fixtures

Add AAIB and IDF schemas to TestHelpers::FinderApi