Unify the test data of AkonadiStorageTest and AkonadiFakeStorageTest

Summary:
We had a slight difference in the test data due to a bug in tag based
item fetches that we chose to ignore. This bug has been fixed, so no
need to have different test data anymore (which in fact now cause a test
failure in shouldListItemsAssociatedWithTag).

Reviewers: bensi, franckarrecot, dfaure, mlaurent, dvratil

Differential Revision: https://phabricator.kde.org/D877