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