play: make the many files warning configurable

[nathdwek]

The warning_treshold key in the play configuration defines the
minimum number of file before a warning is issued (default 100).
A zero value means no warning is ever issued.

Tests are coming. Could you just confirm this is an OK-to-have feature?

[sampsyo]

👍 Looks great.

[nathdwek]

Actually there are no tests for beets-play yet. Do you have any specific advice on what to test and how to test it, since it's an "interoperability" plugin.

[nathdwek]

I added documentation for this feature.

[sampsyo]

Very good question. There are two main approaches we currently use:

• Mocking. Use the mock module to replace the function that calls out to an external command, beets.util.interactive_open, with something that just logs its invocation. Most tests for plugins that use Web services, for example, do some kind of mocking: https://github.com/sampsyo/beets/blob/master/test/test_plexupdate.py#L51
• Dummy command. Make the test configure the plugin to run some trivial command, such as cping the playlist file. The convert plugin does this, for example: https://github.com/sampsyo/beets/blob/master/test/test_convert.py#L40

Do either of those seem reasonable? I'm happy to help if I can!