/kraken/channels/:channel/subscriptions gets a list of people who are subscribed to that channel (channel_subscriptions). The channel obviously needs to have a subscription program for this to work. You cannot really test it without having access to such channel.
/kraken/users/:user/subscriptions/:channel gets user’s subscription to a channel, with the user’s oauth (user_subscriptions).
/kraken/channels/:channel/subscriptions/:user gets user’s subscription to a channel, with the channel’s oauth (channel_subscriptions).