A constant `preload_plugin` is used to disable loading the plugin in the
`setUp` initialisation, allowing the plugin to be loaded manually by the
tests.
Also added a cleanup instruction to remove listeners from configured
plugins, and removed this logic from several tests.
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.
