Unify query creation logic from
- queryparse.py:construct_query_part,
- Model.field_query,
- DefaultTemplateFunctions._tmpl_unique
to a single implementation under `LibModel.field_query` class method.
This method should be used for query resolution for model (flex)fields.
Allow filtering item attributes in album queries and vice versa by
merging `flex_attrs` from Album and Item together as `all_flex_attrs`.
This field is only used for filtering and is discarded after.
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.
