Fix AkonadiStorageTest::shouldNotifyTagAdded() flakiness.

Summary:
On akonadiserver side, if NotificationSubscriber::modifySubscription
happens before NotificationCollector::tagNotification, everything works.
But if it happens the other way around, the notification comes with no
attributes (therefore no "display name").

This solution is the same as what TagTest::testMonitor() in akonadi/autotests/libs/tagtest.cpp

Test Plan:
ctest -R tests-units-akonadi-akonadistoragetest |& less
(and look for shouldNotifyTagAdded)

Reviewers: ervin

Reviewed By: ervin

Subscribers: Zanshin

Differential Revision: https://phabricator.kde.org/D18634