- Fix imports
- Fix pytest issues
- Do not assign lambda as variable
- Use isinstance instead of type to check type
- Rename ambiguously named variables
- Name custom errors with Error suffix
And move the definition to a shared module.
The problem was that EmbedartCliTest ran `_common.TestCase.setUp`
method which initialised temporary directory for the tests AND ran
`helper.TestHelper.setup_beets` method which initialised another set of
temporary directories. This meant that the first set of directories
could not be tracked down for the cleanup.
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.
