Test mails for Decryption Oracle based on replying to PGP or S/MIME.

Summary:
In order to make sure we never add a Decryption Oracle add test mails to
TemplateParser.

BUG: 404698

Reviewers: #kde_pim, aacid, dfaure

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20757