* Replace `noqa` comments in `assert...` method definitions with
a configuration option to ignore these names.
* Use the `__all__` variable to specify importable items from the
module, replacing `*` imports and `noqa` comments for unused imports.
* Address issues with poorly named variables and methods by renaming
them appropriately.
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.
