Decryption Oracle based on forwarding PGP or S/MIME mails (CVE-2019-10732)

Summary:
Add test coverage for mail forwarding.

BUG: 404698

Test Plan:
all tests passes
forward (text/html):
[x] PGP Mime text
[x] PGP Mime html
[x] S/MIME
[x] PGP inline

Reviewers: #kde_pim, vkrause, aacid, dfaure

Subscribers: kde-pim, security-team

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20847