Extra tests for callback processing

[waralex]

From comment on #140 :

If you felt like being extra-paranoid with tests (but I don't think this is necessary for this PR) there are a couple more cases we could in principle cover:

• Both in the single- and multi-output cases, have ALL match only a single element. Note this could be done by creating a new request to the same apps as in the new test and the one you just clarified the name, as we can assume the layout was changed as a result of some other callback.
• A real multi-output callback, where one output is a regular string ID and the other one is an ALL that matches no elements at all, such that the callback should return an empty list for that second output. Note that if it's a single-output callback with just an ALL that matches no elements, the callback will never be triggered.