test(messageviewerplugins): Adopt changes in MessageViewer to rendertest

Summary:
As MessageViewer chagned the way slightly, how mails are rendered, we
need to update the tests here accordingly.

Reviewers: #kde_pim, dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D23863