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.
- generic method to check if operation was performed
- add test of deinterlace operation
- add test for multiple operations performed if required (fails on master)
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.
