Deal with the bug introduced in KCalCore

Summary:
This will get fixed when KCalCore 4.83 will be out, but for the time
being work around it in the test. This is not completely essential for
us.

CCMAIL: montel@kde.org

Reviewers: bensi, franckarrecot, dfaure, mlaurent

Differential Revision: https://phabricator.kde.org/D863