Fix aggregation logic for cacheOnly and ignoreErrors.

Summary:
Add unittests for AggregatedFetch*Scope.
They show that subscribers really need to better clean up after
themselves when unregistering, that's for the next commit (this test is too
low-level for that).

Test Plan: new test

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D18670