This way they get automatically removed with removal of temp_dir.
After this change `test/plugins/test_zero.py` does not any more leave
any mediafield fixtures hanging around.
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.