Clean up test code. No change in functionality.

Removed some redundant module imports, added type signatures for
all top-level functions, and renamed a variable that had been
shadowing another.
Fix typo in ArtistID type

Changed Artist type from "Id Album" to "Id Artist"
Add stub for findRecordings

Currently undefined.
Add tests for Gramophone.Core.Database.findRecordings
Add definition of Gramophone.Core.Database.findRecordings

findRecordings in now implemented.