player: "subprocess" command should stop immediately in idle mode

The description of the "playback_only" field in the "subprocess" command
says "you can't start it outside of playback". This did not work
correctly: if the player was started in idle mode in the first place,
the subprocess was allowed to run even with playback_only=yes.

This is a bug, and this change fixes it. Add a test for this to
command-test.lua.

For #7025.