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.
Beets web API already allows remote players to access audio files but it doesn't provide a way to expose the playlists defined using the smartplaylist plugin.
Now the smartplaylist plugin provides an option to generate ID-based item URIs/URLs instead of paths.
Once playlists are generated this way, they can be served using a regular HTTP server such as nginx.
To provide sufficient flexibility for various ways of integrating beets remotely (e.g. beets API, beets API with context path, AURA API, mopidy resource URI, etc), the new option has been defined as a template with an `$id` placeholder (assuming each remote integration requires a different path schema but they all rely on using the beets item `id` as identifier/path segment).
To prevent local path-related plugin configuration from leaking into a HTTP URL-based playlist generation (invoked with CLI option in addition to the local playlists generated into another directory), setting the new option makes the plugin ignore the other path-related options `prefix`, `relative_to`, `forward_slash` and `urlencode`.
Usage examples:
* `beet splupdate --uri-format 'http://beets:8337/item/$id/file'` (for beets web API)
* `beet splupdate --uri-format 'http://beets:8337/aura/tracks/$id/audio'` (for AURA API)
(While it was already possible to generate playlists containing HTTP URLs previously using the `prefix` option, it did not allow to generate ID-based URLs pointing to the beets web API but required to expose the audio files using a web server directly and refer to them using their file system `$path`.)
Relates to #5037
The boolean flags `--extm3u` and `--no-extm3u` are replaced with a string option `--output=m3u|m3u8`.
This reduces the amount of options and allows to evolve the CLI to support more playlist output formats in the future (e.g. JSON) without polluting the CLI at that point.
