These functions were used to generate mock data for tests but have been
replaced with direct instantiation of AlbumInfo and TrackInfo objects.
This change simplifies the test code and removes unnecessary helper
functions.
External Python packages interfacing beets may want to use an in-memory
beets library instance for testing beets-related code.
The `TestHelper` class is very helpful for this purpose.
Previously `TestHelper` was located in the `test/` directory.
Now it is part of `beets` itself (`beets.test.helper.TestHelper`) and
can be easily imported.
