Fix Attendee UID stabilization for tests

Summary:
Settings this to an empty string doesn't work anymore, as Attendee will
now regenerate a UID in that case. Instead, use the email address like
the other KCalCore tests do.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D21722