Since the `for_artist` keyword has been removed from
`ftintitle.contains_feat`, the unit tests need to be updated.
This includes the deletion of the test cases that test the
`for_artist=True` delimiters.
The previous version of the `plugins.feat_tokens` regular expression
only matched "feat. X" parts if preceded by a space. This caused missed
detections in the `ftintitle.contains_feat` function.
This commit adds unit tests for the updated regex that also matches
"feat. X" parts within parentheses and brackets
- 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
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.
