Before this commit, if we wanted to add channels on rpc initMessaging,
we had to mock this rpc completely.
Now, with this commit, we can use data 'initMessaging', whose keys
can overwrite the default mocked initMessaging.
For instance, if this.data.initMessaging has the key 'channel_slots',
then the mocked initMessaging rpc will return a new state for the
existing channels to the chat_manager.
This is needed in order to reduce the size of tests than need some custom
initMessaging state, such as any test that needs at least one non-static
channel (i.e. neither channel_inbox nor channel_starred).
Alexandre Kühn
22f04b4023